Description of problem:

Many Octavia tempest tests fail because 'octavia' provider driver is not enabled by default in an OSP 15 ML2/OVN deployment. 

Version-Release number of selected component (if applicable):
OSP 15 pre-beta

How reproducible:
100%

Steps to Reproduce:
1. Deploy OSP 15 with ML2/OVN
2. Run Octavia tempest tests, for example octavia_tempest_plugin.tests.scenario.v2.test_l7rule.L7RuleScenarioTest
3. Test fails with "Provider 'octavia' is not enabled."

Actual results:

testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tempest/test.py", line 173, in setUpClass
    six.reraise(etype, value, trace)
  File "/usr/lib/python3.6/site-packages/six.py", line 675, in reraise
    raise value
  File "/usr/lib/python3.6/site-packages/tempest/test.py", line 166, in setUpClass
    cls.resource_setup()
  File "/usr/lib/python3.6/site-packages/octavia_tempest_plugin/tests/scenario/v2/test_l7rule.py", line 51, in resource_setup
    lb = cls.mem_lb_client.create_loadbalancer(**lb_kwargs)
  File "/usr/lib/python3.6/site-packages/octavia_tempest_plugin/services/load_balancer/v2/loadbalancer_client.py", line 91, in create_loadbalancer
    return self._create_object(**kwargs)
  File "/usr/lib/python3.6/site-packages/octavia_tempest_plugin/services/load_balancer/v2/base_client.py", line 101, in _create_object
    response, body = self.post(request_uri, jsonutils.dumps(obj_dict))
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/rest_client.py", line 283, in post
    return self.request('POST', url, extra_headers, headers, body, chunked)
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/rest_client.py", line 679, in request
    self._error_checker(resp, resp_body)
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/rest_client.py", line 790, in _error_checker
    raise exceptions.BadRequest(resp_body, resp=resp)
tempest.lib.exceptions.BadRequest: Bad request
Details: {'faultcode': 'Client', 'faultstring': "Provider 'octavia' is not enabled.", 'debuginfo': None}