Set <host> in cinder.conf to rbd:<pool> for RBD backend

Cinder volume can only be managed by a cinder-volume process that has <host>
configuration variable value matching <os-vol-host-attr:host> attribute of the
volume.

By default, host of a cinder-volume service is set to its hostname, which means
that if one controller is lost, none of its volumes can be managed by
cinder-volume processes on other controllers. This makes sense for Cinder LVM,
but not for RBD or other shared backends.

This commit sets <host> to the same value on all controllers when RBD backend
is enabled, so that any controller can manage RBD backed volumes created by any
other controller.

Change-Id: I47161796dbb817eab026402760799bee24371863
Partial-Bug: #1371328
Merge "Set <host> in cinder.conf to rbd:<pool> for RBD backend" into stable/5.0