Migrate to override_role for network security group tests

Now that override_role has supplanted switch_role (which has
been deprecated) in [0], the RBAC tests need to switch to use
override_role.

This PS switches to override_role for the network security group
test in test_security_groups_rbac.py.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I960c7fb10b7be2f2dd0422a4986ba12d5ecffa75
1 file changed