OVN functional test_impl_idl tests fail if OVN/OVS schemas are not in default locations

If OVN database schemas are not in /usr/local/share/ovn/ location, the tests are gonna fail and there is no way how to override it.

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Failures during discovery
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--- import errors ---
Failed to import test module: networking_ovn.tests.functional.test_impl_idl
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/fixtures/fixture.py", line 197, in setUp
    self._setUp()
  File "/usr/local/lib/python3.6/site-packages/ovsdbapp/venv.py", line 80, in _setUp
    self.setup_dbs()
  File "/usr/local/lib/python3.6/site-packages/ovsdbapp/venv.py", line 166, in setup_dbs
    self.create_db('ovnsb.db', self.ovnsb_schema)
  File "/usr/local/lib/python3.6/site-packages/ovsdbapp/venv.py", line 111, in create_db
    return self.call(['ovsdb-tool', '-v', 'create', name, schema])
  File "/usr/local/lib/python3.6/site-packages/ovsdbapp/venv.py", line 116, in call
    cmd, *args, env=self.env, cwd=cwd, **kwargs)
  File "/usr/lib64/python3.6/subprocess.py", line 311, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['ovsdb-tool', '-v', 'create', 'ovnsb.db', '/usr/local/share/ovn/ovn-sb.ovsschema']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/unittest2/loader.py", line 456, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.6/site-packages/unittest2/loader.py", line 395, in _get_module_from_name
    __import__(name)
  File "/home/cloud-user/networking-ovn/networking_ovn/tests/functional/test_impl_idl.py", line 18, in <module>
    from ovsdbapp.tests.functional import base
  File "/usr/local/lib/python3.6/site-packages/ovsdbapp/tests/functional/base.py", line 23, in <module>
    class FunctionalTestCase(base.TestCase):
  File "/usr/local/lib/python3.6/site-packages/ovsdbapp/tests/functional/base.py", line 30, in FunctionalTestCase
    ovsvenv.setUp()
  File "/usr/lib/python3.6/site-packages/fixtures/fixture.py", line 212, in setUp
    raise MultipleExceptions(*errors)
testtools.runtest.MultipleExceptions: ((<class 'subprocess.CalledProcessError'>, CalledProcessError(1, ['ovsdb-tool', '-v', 'create', 'ovnsb.db', '/usr/local/share/ovn/ovn-sb.ovsschema']), <traceback object at 0x7fc6c1623908>), (<class 'fixtures.fixture.SetupError'>, SetupError({},), <traceback object at 0x7fc6c1623888>))

================================================================================
The above traceback was encountered during test discovery which imports all the found test modules in the specified test_path.
ERROR: InvocationError for command /home/cloud-user/networking-ovn/tools/ostestr_compat_shim.sh (exited with code 100)
___________________________________ summary ____________________________________
ERROR: dsvm-functional: commands failed