oslo.db tries to delete non-existent databases before tests

1. The database name is a random string, so shouldn't already exist.
2. If it *does* already exist, we quite likely want to throw an error
   rather than just delete it.
3. "drop database if exists non_existant_database" throws an error with
   mysqlconnector and a noisy-but-harmless warning with others.

(We can fix the mysqlconnector error in (3), but it is minor compared to (1) and (2))

For reference, the error from (3) with mysqlconnector:
running=${PYTHON:-python} -m oslo.db.sqlalchemy.provision create 8
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File
  "<tox env>/oslo/db/sqlalchemy/provision.py",
  line 153, in <module>
    main()
  File
  "<tox env>/oslo/db/sqlalchemy/provision.py",
  line 146, in main
    print(create_database(engine))
  File
  "<tox env>/oslo/db/sqlalchemy/provision.py",
  line 75, in create_database
    _execute_sql(engine, sqls, driver)
  File
  "<tox env>/oslo/db/sqlalchemy/provision.py",
  line 52, in _execute_sql
    conn.execute(s)
  File
  "<tox env>/sqlalchemy/engine/base.py",
  line 719, in execute
    return self._execute_text(object, multiparams, params)
  File
  "<tox env>/sqlalchemy/engine/base.py",
  line 868, in _execute_text
    statement, parameters
  File
  "<tox env>/sqlalchemy/engine/base.py",
  line 954, in _execute_context
    context)
  File
  "<tox env>/sqlalchemy/engine/base.py",
  line 1116, in _handle_dbapi_exception
    exc_info
  File
  "<tox env>/sqlalchemy/util/compat.py",
  line 189, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File
  "<tox env>/sqlalchemy/engine/base.py",
  line 947, in _execute_context
    context)
  File
  "<tox env>/sqlalchemy/engine/default.py",
  line 435, in do_execute
    cursor.execute(statement, parameters)
  File
  "<tox env>/mysql/connector/cursor.py",
  line 491, in execute
    self._handle_result(self._connection.cmd_query(stmt))
  File
  "<tox env>/mysql/connector/cursor.py",
  line 419, in _handle_result
    self._handle_noresultset(result)
  File
  "<tox env>/mysql/connector/cursor.py",
  line 389, in _handle_noresultset
    self._warnings = self._fetch_warnings()
  File
  "<tox env>/mysql/connector/cursor.py",
  line 709, in _fetch_warnings
    raise errors.get_mysql_exception(res[0][1], res[0][2])
sqlalchemy.exc.DatabaseError: (DatabaseError) 1008: Can't drop database
'vgddrfgfsm'; database doesn't exist 'drop database if exists
vgddrfgfsm;' {}