Migrate to override_role for network multiprovider 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_networks_multiprovider_rbac.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: Id1e8da294c853be5408f39fc032aefe30abeea93
1 file changed