VALID_PLAN_STATES should contain :update_rollback_complete and :update_rollback_failed

https://github.com/miasma-rb/miasma/blob/develop/lib/miasma/models/orchestration/stack.rb#L10-L13

I ran into a problem with not able to update a stack when it was in update_rollback_complete and update_rollback_failed state. Would you accept a PR to add them into the VALID_PLAN_STATES array?

@chrisroberts 