add InstanceGroup.get_by_instance_uuid

Add a new method to the InstanceGroup object that lets you get a group
by an instance UUID.  Right now Nova only lets you have an instance in
a single group, so that's why this is a method of InstanceGroup and
not InstanceGroupList.

Change-Id: Ic0181e241471ad817284209e6a1093a438962bf1
Related-bug: #1379451
Merge "add InstanceGroup.get_by_instance_uuid"