mon/MDSMonitor: Cope with failures during creation

Previously an MDS which failed in state CREATING could be
assigned the sequence BOOT->STANDBY->REPLAY at its next
startup, which fails to re-try the creation.

Fixes: #7485

Signed-off-by: John Spray <john.spray@inktank.com>
mds: add mds_kill_create_at for testing

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1283 from ceph/wip-7485

mon/MDSMonitor: Cope with failures during creation

Reviewed-by: Greg Farnum <greg@inktank.com