gluster: Fix "key" attribute for gluster volumes

gluster: Fix "key" attribute for gluster volumes

According to our documentation the "key" value has the following
meaning: "Providing an identifier for the volume which identifies a
single volume." The currently used keys for gluster volumes consist of
the gluster volume name and file path. This can't be considered unique
as a different storage server can serve a volume with the same name.

Unfortunately I wasn't able to figure out a way to retrieve the gluster
volume UUID which would avoid the possibility of having two distinct
keys identifying a single volume.

Use the full URI as the key for the volume to avoid the more critical
ambiguity problem and document the possible change to UUID.