Fix RBD with external Ceph without Cephx use case

When a rbd_user is set in the cinder.conf, Nova will re-use
that username in order to look up a libvirt secret, which fails,
even though none is stored and none is needed.

Make it more explicit that the user name needs to be emptied
and removed from the cinder config (set to default "None")
Code style fix
Merge pull request #142 from dirkmueller/release/roxy/master

Fix RBD with external Ceph without Cephx use case