mon/Paxos: bootstrap peon too if monmap updates

If we get a monmap update, the leader bootstraps.  Peons should do the
same.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit efe5b67bb700ef6218d9579abf43cc9ecf25ef52)