Group Policy API-1: EP, EPG, L2 Context, L3 Context

This is the first in a series of patches which implement Group Policy. It
includes the Resource Model and API for Endpoint, Endpoint Group, L2
Context, and L3 Context resources.

In the context of the larger Group Policy Model, the Endpoint Group resource
references Contracts, which will be introduced in a subsequent patch.

The DB and Plugin layers to back these resources will also be introduced in
subsequent patches.

Gerrit Spec: https://review.openstack.org/#/c/89469
("bridge_domains" and "routing_domains" resources in the above spec have been
renamed to "l2_contexts" and "l3_contexts"; "ip_supernet" and
"subnet_prefix_length" attributes in "routing_domains" has been renamed to
"ip_pool" and "default_subnet_prefix_length" attributes in "l3_contexts")

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>