Use model_base from neutron_lib

This follows the commit 60ae70dde196191217c8c46242be283c7fd30039.
That code has been approved upstream

Change-Id: Ibe283886cf81822750db4b1dedfa69cdac1f7e43

vmware_nsx/db/extended_security_group.py

@@ -13,6 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from neutron_lib.db import model_base
 from oslo_utils import uuidutils
 import sqlalchemy as sa
 from sqlalchemy import orm
@@ -21,7 +22,6 @@ from neutron.api.v2 import attributes
 from neutron.common import utils as n_utils
 from neutron.db import api as db_api
 from neutron.db import db_base_plugin_v2
-from neutron.db import model_base
 from neutron.db.models import securitygroup as securitygroups_db  # noqa
 from neutron.extensions import securitygroup as ext_sg
 from neutron_lib.api import validators

vmware_nsx/db/extended_security_group_rule.py

@@ -13,11 +13,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from neutron_lib.db import model_base
 import sqlalchemy as sa
 from sqlalchemy import orm
 
 from neutron.db import db_base_plugin_v2
-from neutron.db import model_base
 from neutron.db import securitygroups_db
 from neutron.extensions import securitygroup as ext_sg
 from neutron_lib.api import validators

vmware_nsx/db/migration/alembic_migrations/env.py

@@ -15,6 +15,7 @@
 from logging import config as logging_config
 
 from alembic import context
+from neutron_lib.db import model_base
 from oslo_config import cfg
 from oslo_db.sqlalchemy import session
 import sqlalchemy as sa
@@ -22,7 +23,6 @@ from sqlalchemy import event
 
 from neutron.db.migration.alembic_migrations import external
 from neutron.db.migration.models import head  # noqa
-from neutron.db import model_base
 
 from vmware_nsx.db.migration import alembic_migrations
 

vmware_nsx/db/nsx_models.py

@@ -19,11 +19,11 @@ This module defines data models used by the VMware NSX plugin family.
 
 """
 
+from neutron_lib.db import model_base
 import sqlalchemy as sa
 from sqlalchemy import orm
 from sqlalchemy import sql
 
-from neutron.db import model_base
 from neutron.db import models_v2
 from oslo_db.sqlalchemy import models
 

vmware_nsx/db/nsxv_models.py

@@ -14,12 +14,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-
+from neutron_lib.db import model_base
 import sqlalchemy as sa
 from sqlalchemy import orm
 
 from neutron.db import l3_db
-from neutron.db import model_base
 from neutron.db import models_v2
 from oslo_db.sqlalchemy import models
 

vmware_nsx/db/vcns_models.py

@@ -14,10 +14,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-
+from neutron_lib.db import model_base
 import sqlalchemy as sa
 
-from neutron.db import model_base
 from neutron.db import models_v2
 from oslo_db.sqlalchemy import models