Migrate to override_role for network test_networks_rbac

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_networks_rbac.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I8a90f63aa488d37a1b779ab1d0c00c031c356e81
1 file changed