mon: do not force proposal when no osds

If there are no OSDs, there is no need to propose to paxos. It does not
hurt on a production cluster but it matters when running tests because
it effectively alway ignores --paxos-propose-interval.

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>