Add Multi Backend support

This is based on previous work by Sascha and with help from
Balazs, Vincent and Thomas.
Allow using different ceph deployments for ceph backends

There can be only one Ceph deployed by Crowbar, but other deployments
can also be used.
Start cinder-volume after we're done with everything

We were trying to start it too early, including before enabling loop.lvm
which is required to be enabled before we can start cinder-volume.
Only include ceph::cinder recipe on nodes with the volume role
Require confirmation before deletion of a backend
Merge pull request #197 from dirkmueller/multi_backend

Add Multi Backend support