mds: Handle setting 'enabled' in upgrades

Previously was always defaulting to disabled,
should be enabled if MDS map epoch is >1 (i.e.
if an MDS was ever started).

Signed-off-by: John Spray <john.spray@redhat.com>
mon/MDSMonitor: s/enabled/get_enabled()/

Signed-off-by: John Spray <john.spray@redhat.com>
mon/MDSMonitor: fix incorrect comment

Signed-off-by: John Spray <john.spray@redhat.com>
mon/MDSMonitor: log warning while MDS up but no FS

Signed-off-by: John Spray <john.spray@redhat.com>