Improve documentation around usage of cinder-volume-nfs

This change aims at improving the doc around usage of the different
cinder backend elements, particularily cinder-volume-nfs.

We also remove cinder-volume from the BLOCKSTORAGE image elements as
it will be eventually installed as dependency by one of the
cinder-* backend elements.

Change-Id: Id553b15227c0b980702174d7d2a0410ab30dcce4
Merge "Improve documentation around usage of cinder-volume-nfs"