mon: MonmapMonitor: make 'ceph mon add' idempotent

MonMap changes lead to bootstraps.  Callbacks waiting for a proposal to
finish can have several fates, depending on what happens: finished, rerun
or aborted.

In the case of a bootstrap right after a monmap change, callbacks are
rerun.  Considering we queued the message that lead to the monmap change
on this queue, if we instead of finishing it end up reruning it, we will
end up trying to perform the same modification twice -- the last one will
try to modify an already existing state and we will return just that:
whatever you're attempting to do has already been done.

This patch makes 'ceph mon add' completely idempotent.  If one tries to
add an already existing monitor (i.e., same name, same ip:port), one
simply gets a 'monitor foo added', with return 0, no matter how many
times one runs the command.

Fixes: #5896

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