Dynamic Management of Server Groups: Remove VM

The primary objective of this blueprint is to extend the server group (formerly known as "instance groups") APIs such that VMs can be removed from the group. At present, a VM can only be removed from a group by deleting the respective VM. This puts cloud administrators in a tough position if a VM is still needed, but its affinity rule is not.

To that end, this blueprint capitalizes on the foundation already in place (i.e., namely the related affinity rules within the scheduler) and adds some incremental API enhancements that make server groups much more consumable from an end-user perspective. The "remove" case is trivial since there are no policy violations or race conditions to consider; yet it still provides value as the user need not delete their VM to "undo" an affinity rule in the event it's no longer applicable.

Primary Use Case:
* As a cloud administrator, I want to remove a VM from a server group (i.e., without having to delete the VM), so that I can update my groups as affinity rules change over time.

There is also a related blueprint for adding a VM to a server group [1].

[1] https://blueprints.launchpad.net/nova/+spec/dynamic-server-groups-add