Add RBAC access_as_external unit tests
Add RBAC access_as_external unit tests

Currently, most of the RBAC tests in [1] are related to shared.
We can add some tests to cover the access as external situation.

[1] https://github.com/openstack/neutron/blob/master/neutron/tests/unit/db/test_rbac_db_mixin.py

Change-Id: I2d17a8f35d3289b2dd7ee0a78bc8b196f549c442
Closes-Bug: #1726677

noironetworks/neutron,mahak/neutron,openstack/neutron,mahak/neutron,eayunstack/neutron,mahak/neutron,eayunstack/neutron,huntxu/neutron,noironetworks/neutron,huntxu/neutron,openstack/neutron,openstack/neutron