Fails to build with latest neutron

[Joel Capitao]

DEBUG: =========================
DEBUG: Failures during discovery
DEBUG: =========================
DEBUG: --- import errors ---
DEBUG: Failed to import test module: neutron_taas.tests.unit.db.test_migrations
DEBUG: Traceback (most recent call last):
DEBUG: File "/usr/lib64/python3.9/unittest/loader.py", line 436, in _find_test_path
DEBUG: module = self._get_module_from_name(name)
DEBUG: File "/usr/lib64/python3.9/unittest/loader.py", line 377, in _get_module_from_name
DEBUG: __import__(name)
DEBUG: File "/builddir/build/BUILD/tap-as-a-service-14.1.0.dev11/neutron_taas/tests/unit/db/test_migrations.py", line 30, in <module>
DEBUG: class _TestModelsMigrationsTAAS(test_migrations._TestModelsMigrations):
DEBUG: AttributeError: module 'neutron.tests.functional.db.test_migrations' has no attribute '_TestModelsMigrations'
DEBUG: ================================================================================
DEBUG: The above traceback was encountered during test discovery which imports all the found test modules in the specified test_path.

The neutron patch [1] seems to be the reason, taas needs to update the unit test.

https://review.opendev.org/c/openstack/neutron/+/934171

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/tap-as-a-service/+/935147