This patch adds support for external Ceph clusters for Cinder.

For clean integration the backend configuration mechanism had to be
slightly adjusted.

We now have the option to enable multiple backends for Cinder
independently.

Currently, the flags cinder_backend_iscsi and cinder_backend_ceph are
used to toggle backends.

Documentation on how to use external ceph was added.

Change-Id: I7e0267b90d62d6d881f24f063cdb894422ec8618
Partially-Implements: Blueprint: external-ceph