Use function registration for policy checks

Use function registration for policy checks

The original policy framework allowed new policy checks to be created
through inheritance.  This is somewhat clunky and unnecessary in
Python.  This change refactors policy.py to allow new policy checks
to be registered using an @register() decorator.  One consequence is
that HttpBrain is deprecated.

Care has been taken to ensure backwards compatibility; deprecation
warnings will be emitted for uses of HttpBrain or the inheritance-
based checks.

(Pull-up from openstack-common, with deprecation of HttpBrain.)

Change-Id: I0aa78d1128e55aa7d59558c62128105c01da8f1b