mon: Monitor: use rank instead of name when randomly picking monitors

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: fix bug on _pick_random_mon() that would choose an invalid rank

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge remote-tracking branch 'gh/wip-mon-rank' into next

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>