Add REST API for instance group api extension

Support the Creation, Read, Delete, and List of server groups.

Refactored the code to use objects (https://review.openstack.org/#/c/38979/
Renamed from "instance group" to "server group".

Implements: blueprint instance-group-api-extension

Change-Id: I650a8f191dea5eab5b4b1828f0b9f65e33edea2a