When deploying for HA, set cinder's backend_host option (not the host
option) to "hostgroup". That way, only the cinder-volume backend drivers
use a common value.
Closes-Bug: #1753596
Depends-On: I78a88725d297794efd854fbfb14df0611e0e0738
Change-Id: Ifa4cd1f43fd5d73fc21630cee45b9f38fa882349
(cherry picked from commit 99ea8b681f)