Add a OS::Nova::ServerGroup resource.

Change-Id: Ia6dc596b3ddcfb29dfe8f712794584fb2f6954ca
Implements: blueprint vm-instance-group-support
Updated from global requirements

Change-Id: I8947fe9a024ed9ac546473b21546584961b422e8
Merge "Add a OS::Nova::ServerGroup resource."
Merge "Updated from global requirements"