Migrate to override_role for network metering 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_metering_label_rules_rbac.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I84177b1c9384be54ae62203118ee80eea11f24a8
1 file changed