Group Policy Model - Part 1

This is the first in a series of patches which implement Group Policy. It
includes Group Policy API, and Resource Model with DB implementation. This
patch contains the resources which are required in the Group Policy Model.
Follow up patches will have the mapping to the Neutron model, and also the
optional resources.

Gerrit Spec: https://review.openstack.org/#/c/89469

Implements: blueprint group-based-policy-abstraction

Author:    Sumit Naiksatam <sumitnaiksatam@gmail.com>
Author:    Bob Kukura <kukura@noironetworks.com>
Author:    Stephen Wong <s3wong@midokura.com>
Author:    Mohammad Banikazemi <mb@us.ibm.com>
Author:    Mandeep Dhami <dhami@noironetworks.com>