Fix ipam_pluggable_backend unit tests module

This module couldn't be run alone due to missing registratration of
the "ipam_driver" config option from the core neutron configuration.
To solve that problem there is test plugin configured for those tests
and it registers that option which is necessary.

Change-Id: Iffdca967340c01d4e8c063c516d0d5441750c653
(cherry picked from commit a3e68e8)

Author: slawqo

neutron/tests/unit/db/test_ipam_pluggable_backend.py

@@ -63,9 +63,11 @@ class TestIpamSubnetPool(UseIpamMixin, test_db_base.TestSubnetPoolsV2):
 
 
 class TestDbBasePluginIpam(test_db_base.NeutronDbPluginV2TestCase):
-    def setUp(self):
+    def setUp(self, plugin=None):
+        if not plugin:
+            plugin = 'neutron.tests.unit.db.test_ipam_backend_mixin.TestPlugin'
+        super(TestDbBasePluginIpam, self).setUp(plugin=plugin)
         cfg.CONF.set_override("ipam_driver", 'internal')
-        super(TestDbBasePluginIpam, self).setUp()
         self.tenant_id = uuidutils.generate_uuid()
         self.subnet_id = uuidutils.generate_uuid()
         self.admin_context = ncontext.get_admin_context()
@@ -413,7 +415,7 @@ def test_create_ipv6_pd_subnet_over_ipam(self, pool_mock):
                          subnetpool_id=constants.IPV6_PD_POOL_ID,
                          ipv6_ra_mode=constants.IPV6_SLAAC,
                          ipv6_address_mode=constants.IPV6_SLAAC):
-            self.assertEqual(2, pool_mock.get_instance.call_count)
+            self.assertEqual(3, pool_mock.get_instance.call_count)
             self.assertTrue(mocks['driver'].allocate_subnet.called)
             request = mocks['driver'].allocate_subnet.call_args[0][0]
             self.assertIsInstance(request, ipam_req.SpecificSubnetRequest)