block/gluster: rename [server, volname, image] -> [host, volume, path]

block/gluster: rename [server, volname, image] -> [host, volume, path]

A future patch will add support for multiple gluster servers. Existing
terminology is a bit unusual in relation to what names are used by
other networked devices, and doesn't map very well to the terminology
we expect to use for multiple servers.  Therefore, rename the following
options:
'server'  -> 'host'
'image'   -> 'path'
'volname' -> 'volume'

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1468947453-5433-2-git-send-email-prasanna.kalever@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>