tests failed when running tox without `--concurrency 1`

Passage of the tests depends on the order of their execution so we can't run tests in parallel.

Sample error output:
======================================================================
FAIL: glance.tests.unit.test_swift_store.TestStoreAuthV2.test_add_large_object_zero_size
tags: worker-2
----------------------------------------------------------------------
Traceback (most recent call last):
  File "glance/tests/unit/test_swift_store.py", line 614, in test_add_large_object_zero_size
    self.assertEqual(expected_location, location)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 348, in assertEqual
    self.assertThat(observed, matcher, message)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/testtools/testcase.py", line 433, in assertThat
    raise mismatch_error
MismatchError: !=:
reference = 'swift+config://ref1/glance/170ff390-bf41-443f-9cbd-a29cbdd36785'
actual = 'swift+https://tenant%3Auser1:key@localhost:8080/glance/170ff390-bf41-443f-9cbd-a29cbdd36785'

======================================================================
FAIL: glance.tests.unit.test_migrations.TestMigrations.test_walk_versions
tags: worker-1
----------------------------------------------------------------------
Traceback (most recent call last):
  File "glance/tests/unit/test_migrations.py", line 251, in test_walk_versions
    self._walk_versions(engine, self.snake_walk)
  File "glance/tests/unit/test_migrations.py", line 344, in _walk_versions
    self._migrate_down(engine, version - 1, with_data=True)
  File "glance/tests/unit/test_migrations.py", line 365, in _migrate_down
    version)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/versioning/api.py", line 202, in downgrade
    return _migrate(url, repository, version, upgrade=False, err=err, **opts)
  File "<string>", line 2, in _migrate
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/versioning/util/__init__.py", line 160, in with_engine
    return f(*a, **kw)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/versioning/api.py", line 366, in _migrate
    schema.runchange(ver, change, changeset.step)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/versioning/schema.py", line 93, in runchange
    change.run(self.engine, step)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/versioning/script/py.py", line 148, in run
    script_func(engine)
  File "/home/ochuprykov/glance/glance/db/sqlalchemy/migrate_repo/versions/004_add_checksum.py", line 84, in downgrade
    images.columns['checksum'].drop()
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/changeset/schema.py", line 592, in drop
    engine._run_visitor(visitorcallable, self, connection, **kwargs)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1617, in _run_visitor
    conn._run_visitor(visitorcallable, element, **kwargs)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1246, in _run_visitor
    **kwargs).traverse_single(element)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/changeset/ansisql.py", line 56, in traverse_single
    ret = super(AlterTableVisitor, self).traverse_single(elem)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/visitors.py", line 120, in traverse_single
    return meth(obj, **kw)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/changeset/databases/sqlite.py", line 135, in visit_column
    super(SQLiteColumnDropper,self).visit_column(column)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/changeset/databases/sqlite.py", line 98, in visit_column
    self.recreate_table(table,column,delta)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/migrate/changeset/databases/sqlite.py", line 85, in recreate_table
    table.create(bind=self.connection)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/schema.py", line 655, in create
    checkfirst=checkfirst)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1246, in _run_visitor
    **kwargs).traverse_single(element)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/visitors.py", line 120, in traverse_single
    return meth(obj, **kw)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 736, in visit_table
    self.traverse_single(index)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/visitors.py", line 120, in traverse_single
    return meth(obj, **kw)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 748, in visit_index
    self.connection.execute(CreateIndex(index))
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 729, in execute
    return meth(self, multiparams, params)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 69, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 783, in _execute_ddl
    compiled
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 958, in _execute_context
    context)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1160, in _handle_dbapi_exception
    exc_info
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 951, in _execute_context
    context)
  File "/home/ochuprykov/glance/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 436, in do_execute
    cursor.execute(statement, parameters)
OperationalError: (OperationalError) disk I/O error u'CREATE INDEX ix_images_is_public ON images (is_public)' ()