Fix glusterfs volume creation

Fix the following error message with glusterfs on Trusty:
    volume create: medias_volume_0: failed: The brick
    oscied-local-machine-5:/mnt/bricks/exp0 is is being created in the
    root partition. It is recommended that you don't use the system's root
    partition for storage backend. Or use 'force' at the end of the command
    if you want to override this behavior