Fixed Ceph backup librbd segfault

Change-Id: I1a6b9935eb7da9fca5b572ea4823bae3def99ea0
Fixes: bug #1198271
Check enabled backup service before rpc request

In previous code we didn't check whether we have an enabled
backup service there before we send out the rpc request,
results that if no enabled backup service there,the volume will
stays in "backing-up" state and the backup will stays in "creating".

This patch fixed this issue, we exam whether at least an appropriate
(same host,az as the volume and is_alive) backup service available
before we do the rpc request.

Fixes: bug #1200040
Change-Id: I77154528f489ed20f7b784e6fcefccf15dc81d1d
Merge "Fixed Ceph backup librbd segfault"
Merge "Check enabled backup service before rpc request"