Add the ability to run the cinder-volume service in an active-active
configuration. This involves two settings:
1) Configure the service's cluster name. This is the signal to the
cinder-volume service that it should run in active-active mode.
2) Configure the "backend URL" cinder uses for its distributed lock
manager (DLM). This patch assumes that DLM is etcd3, and it's
accessible via an HTTP gateway.
Depends-On: Ib37c75952fed7c762a71dae3cd169b7753faf6f7
Change-Id: I615af64086d46356f322094d9f3b4e29557ed899