Refactor InstanceGroup to use a nested stack

Much of the code of InstanceGroup was able to be deleted in
preference for using the native functionality of StackResource.
InstanceGroup now generates a template and updates the nested
stack when it needs to be created or resized.

blueprint instance-group-nested-stack

Fixes bug 1189278

Change-Id: Ic08a55cad1ac34d69080c0ef2dae4877f1fefd29
Merge "Refactor InstanceGroup to use a nested stack"