mon: Monitor: make 'quorum enter/exit' available through the admin socket

If we don't have at least 'quorum enter' available through the admin
socket, then we can never tell a monitor to go back into quorum after
exiting, since it will not handle the command that forces it back in due
to not being in the quorum.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
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.

Fixes: #6705

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #835 from ceph/wip-6705

qa: workunits: mon: ping.py: test 'ceph ping'

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>