Add expect bootstrap '-expect=n' mode.

This allows for us to automatically bootstrap a cluster of nodes after
'n' number of server nodes join. All servers must have the same 'n' set, or
they will fail to join the cluster; all servers will not join the peer set
until they hit 'n' server nodes.

If the raft commit index is not empty, '-expect=n' does nothing because it
thinks you've already bootstrapped.

[comments/criticisms/style tips/life tips appreciated]
Okay, so this is what we need to change. Basically, the problem here is that all nodes are operating with EnableSingleNode, and roughly at the same time, each server is attempting to do an AddPeer to all other peers, which will result in chaos. Will discuss on IRC