ResourceGroup update refactor

Reworks ResourceGroup.handle_update a little so we save the updated
properties in the resource object before processing the update.

This is required so we can have a hint property which specifies
victims to be removed on update

Change-Id: I4884a929ede1c6bff835fe789e5dd890a06831c2
blueprint: resourcegroup-force-remove
Support Cinder scheduler hints

Add a 'scheduler_hints' option for OS::Cinder::Volume objects, as is
it already done for OS::Nova::Server.

Implements: blueprint cinder-scheduler-hints
Change-Id: I090359c212df1d17b10e374d2e80e22344017aee
Merge "Support Cinder scheduler hints"
Merge "ResourceGroup update refactor"