demo: do not use numerical value for MDS
demo: do not use numerical value for MDS

Only jewel,kraken,luminous support a numerical value as the MDS name. As
for Mimic this is deprecated and requires a full name so let's use the
  default MDS_NAME variable.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/ceph-docker,ceph/ceph-docker