mon: include PGMonitor commands with mixed-version mons

mon: include PGMonitor commands with mixed-version mons

While we have a mixed version cluster, we have to advertise our
PGMonitor commands to our peons or else commands like 'pg dump'
won't work.

Once the mon feature flag is set, we can drop that because each
mon will include the mgr commands (either those stored in paxos
or the statically compiled ones until that point).

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 2d9045a9a57a3b08d525f4f1ecb3b255add26b1f)