Utilize the "ringbuilder.devs_changed" option to ensure that the ring
needs a rebalance. This will prevent unnecessary rebalances that cause
failures due to "min_part_hours" not being passed even though no changes
were required.
Additionally, we can now return a correct Ansible repsonse when the ring
has changed/rebalanced - and return "OK" when it hasn't returned at all.
Change-Id: I1fb4b3544a50ab5f566b3846d616107a84ff29c9