Support Nova ServerGroup

[ezeriver94]

What would you like to see implemented?

I would like ORC to provide functionality to manage Compute ServerGroups.

Additional information

I'm open for implementing it myself if this is not part of your current path

Link to reference documentation

https://docs.openstack.org/nova/latest/user/server-groups.html

[mandre]

It's up for grab if you want to give it a shot.

[ezeriver94]

created PR #406