Use config fixture

Use the configuration fixture instead of the old opt_in_group and
opt methods.

Where possible the config_fixture usage done from within setUp
has been moved to config_overrides() method. Notably a number of
the Policy.json tests cannot be moved into config_overrides
due to the need to control how and when policy data is loaded.

Change-Id: I1662627cca30890cc3f3da83772132cec7bd58e0
Merge "Use config fixture"