Group policy API-2 resources: Classifiers, Actions and 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: Iaff15a534f7ac6b979c8b783907705406ee2cbb2
Implements: blueprint neutron-grouppolicy
Group Policy API-3: Contracts

This is the third patch in the Group Policy resources implementation
series. This patch implements:
Contracts

In the context of larger Group Policy model, Contracts contain Policy
Rules and Endpoint Groups consume or provide Contracts.

Change-Id: If12f966ac360d2dd9f459d720fc4114e7ffc6e9e
Implements: blueprint neutron-grouppolicy