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I'm looking to add support for server groups to heat.  I've got working 
code, but I thought I'd post the overall design here in case people had 
objections.

Basically, what I propose is to add a "class NovaServerGroup" resource. 
  Currently it would only support a "policy" property to store the 
scheduler policy for the server group.  The scheduler policy would not 
support updating on the fly.

The "LaunchConfiguration" and "Instance" classes would be extended with 
an optional "ServerGroup" property.  In the "Instance" class if the 
"ServerGroup" property is set then the group name is added to the 
scheduler_hints when building the instance.

The "Server" class would be extended with an optional "server_group" 
property.  If it is set then the group name is added to the 
scheduler_hints when building the server.

All in all, its around a hundred lines of code.

Any comments?

Chris