Initial scheduler support for instance_groups

Scheduler support for blueprint instance-group-api-extension

The scheduling code now uses the instance group object. This
patch set adds support for 'anti-affinity' and 'affinity'
policies.

Co-authored-by: Russell Bryant <rbryant@redhat.com>
Change-Id: I539424192727cf2a768d6b24ba9cf36dc56c9304