Fix SQLAlchemy warnings with view only relationships

The view only relatinships must be specified in the DB model.
The following are fixed:
- QosNetworkPolicyBinding.port: "port" DB object will have a view only
  copy of the "network" QoS policy.
- Subnet.rbac_entries: "subnet" DB object will have a view only copy
  of the "network" RBAC policies.

Change-Id: I2396c0a23b00439d46faf4b29a082b3819528d89
Closes-Bug: #1927763

neutron/db/models_v2.py

@@ -230,7 +230,8 @@ class Subnet(standard_attr.HasStandardAttributes, model_base.BASEV2,
     rbac_entries = orm.relationship(
         rbac_db_models.NetworkRBAC, lazy='subquery', uselist=True,
         foreign_keys='Subnet.network_id',
-        primaryjoin='Subnet.network_id==NetworkRBAC.object_id')
+        primaryjoin='Subnet.network_id==NetworkRBAC.object_id',
+        viewonly=True)
     api_collections = [subnet_def.COLLECTION_NAME]
     collection_resource_map = {subnet_def.COLLECTION_NAME:
                                subnet_def.RESOURCE_NAME}

neutron/db/qos/models.py

@@ -59,8 +59,8 @@ class QosNetworkPolicyBinding(model_base.BASEV2):
         primaryjoin='QosNetworkPolicyBinding.network_id == Port.network_id',
         foreign_keys=network_id,
         backref=sa.orm.backref('qos_network_policy_binding', uselist=False,
-                               viewonly=True, lazy='joined'),
-        sync_backref=False)
+                               lazy='joined'),
+        sync_backref=False, viewonly=True)
 
 
 class QosFIPPolicyBinding(model_base.BASEV2):