mon: do not allow monitors to be added to the map with port 0
Last week, somebody used the "ceph mon add" command without specifying a port, and it defaulted to port 0. This caused all kinds of trouble because the new monitor started up using port 6789 (because it was unspecified), but the other monitors tried to communicate with it on port 0 (because that's what was in the monmap).
Instead, if the port is not specified, set it to the default 6789. Make sure that this does not allow creating two monitors with the same IP in the map (ie, make sure that we don't bypass any error checking by doing that).
We need this to go on current master and DHO.