Group Policy Plugin-2: Classifiers, Actions, and Rules

(Patch series identifier: GP-PLG-2)

This patch includes the plugin and the policy driver framework to realize the
Policy Classifiers, Policy Actions, and Policy Rules resources.

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

Gerrit Spec: https://review.openstack.org/#/c/89469
("classifiers" and "actions" resources in the above spec have been renamed to
"policy_classifiers" and "policy_actions")

Partially-implements: blueprint group-based-policy-abstraction

Change-Id: Ia7d4b379091a36152fde219918348fb2a41577a8
Author: Sumit Naiksatam <sumitnaiksatam@gmail.com>
Co-Authored-By: Bob Kukura <kukura@noironetworks.com>
Co-Authored-By: Stephen Wong <s3wong@midokura.com>
Co-Authored-By: Mohammad Banikazemi <mb@us.ibm.com>
Co-Authored-By: Mandeep Dhami <dhami@noironetworks.com>