mon: limit initial quorum to mon_initial_members

mon: limit initial quorum to mon_initial_members

This is a two-stage process.

 * If we start up, and have never joined a quorum, and initial members are
   specified, only include them in the monmap; put all others in the
   extra probe list.
 * We add missing members if necessary to make the monmap (and initial
   quorum) the right size.
 * If we probe someone that *has* participated in a successful quorum, we
   get their monmap, and restart, so this is moot.
 * We only call an election to create a new cluster if outside_quorum gets
   big enough (it will only include initial members) and if it includes us.

Signed-off-by: Sage Weil <sage@inktank.com>