Merge remote-tracking branch 'origin/wip-15065'

Support new standby_for_fscid option, letting us assign daemons to FS clusters.

Reviewed-by: Greg Farnum <gfarnum@redhat.com>

diff --cc src/common/config_opts.h
Simple merge

diff --cc src/mds/FSMap.cc
Simple merge

diff --cc src/mon/MDSMonitor.cc
Simple merge