Group policy API-2 HEAT resources: Classifiers, Rules

This is the second patch in the Group Policy resources implementation
series. This patch implements:
    Policy Classifiers
    Policy Actions
    Policy Rules

In the context of larger Group Policy model, the Policy Rule resource
is referenced by Contracts, which will be introduced in the subsequent
patch.

Change-Id: If6df98133bb517ebfba37e20a988d9837aeee7f8
Implements: blueprint group-based-policy-automation