BZ #1110504 - nfs-utils missing, cinder can't mount nfs share

https://bugzilla.redhat.com/show_bug.cgi?id=1110504

Ensure nfs-utils is installed when Cinder is using NFS backend.
Ensure Cinder enabled_backends is empty when it should

When switching from `multiple_backends = true` to `multiple_backends =
false`, the `enabled_backends` option in cinder.conf wouldn't get
cleared, and Cinder would still attempt to use multiple backends. This
is now fixed.
Merge pull request #277 from jistr/nfs_utils

nfs-utils installed for cinder, multi -> single backend transition fix