mon: cycle through keyring or key_server for auth with mons

mon: cycle through keyring or key_server for auth with mons

After commit `mon: use key_server for looking up mon key`, the mons will now
use the key_server to lookup the `mon.` key when a mon connects.  We need to
make the mons prefer using that key with authenticating during probing other
mons. However, the protocol doesn't allow falling back to another key. This is
necessary if what's in the key_server database is out-of-date due to an earlier
loss of quorum. In that case, the operator should update the local keyring file
and the mon should give that a try if auth fails.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>