RBD backend: use ensure_resource

In the case we want to declare more than one backend using RBD driver,
we have a resource dupplication which could be fixed by using stlib
functions.

Closes-bug #1294138
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>

Change-Id: I1783540cd8218e8bafb4d0b860ca4c5992f35a19
(cherry picked from commit 6dba575af6f119385614b390c92dc258032a480e)