fix(governor): don't bootstrap vshard until replication factor is satisfied

NOTE: there's still a bug, because we set each new replicaset's weight to 1 (even ones which don't satisfy replication factor) before the bucket distribution is bootstrapped. But there must be at least one replicaset with non zero weight in order for vshard.*.cfg to work.

A potential solution would be to only configure vshard once a replicaset is filled up.

Close #155 (closed)