Add ability to set prefix in SolidFire Volume name

Add ability to set prefix in SolidFire Volume name

The SolidFire creates names for Volumes on the backend using
the format UUID-<cinder-id>

This works well, but it turns out that there is a possibility
of a UUID collision for customers running multiple clouds against
the same cluster. To eliminate that possibility we should allow
a config option to set the prefix to be whatever an admin chooses.

This patch introduces the sf_volume_prefix config option to
address this bug.  The default still uses the 'UUID-' prefix.

DocImpact
Changing this setting on an existing deployment will
result in the existing volumes being inaccessible. To introduce
this change to an existing deployment it's recommende to
add the Cluster as if it were a second backend and disable
new deployments to the current backend.

Change-Id: I21a87e6431099e51355044f2dfea35b61ca8125f
Closes-Bug: #1503069