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]
If EnableSingleNode is not true, we never reach joinConsulServer() in leader.go... Unless I've missed something?