Make a dedicated InstanceGroup

Make it the basis of the autoscaling group, the
main difference is you change the number of instances
via update (manually) or not at all.

This can be achieved using the autoscaling group, but
this is clearer from the user's perspective.

Implements blueprint static-inst-group

Change-Id: I72680e92183ba87a76efa64383269afb083a446a