Migrate to override_role for network port 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 test
test_ports_rbac.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I015ec82431a2d6b6f2346da73f0d4ca901c6c274
1 file changed