qa: workunits: mon: ping.py: test 'ceph ping'

Basic testing by forcing each monitor out of quorum at a time and making
sure they still reply to ping requests.

Also add a 'quorum enter/exit' command to the admin socket interface so
that we can force a monitor to exit or enter the quorum. If we force a
monitor out of the quorum using 'ceph quorum exit', regardless of doing it
through the admin socket, that monitor will not handle the 'quorum enter'
command and will have to be restarted if we want it to start participating
again -- that is due exactly to being out of the quorum!  This additional
patch fixes it.

Fixes: 6705