Migrate to override_role for network metering_labels 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_labels_rbac.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: Iccefa15039b5a2fccdce07aafca073cc79d7b917
1 file changed