etcdserver: Calculate IDs for nodes solely on PeerURLs

Removes the notion of name being anything more than advisory or
command-line grouping, and adds checks for bootstrapping the command
line. IDs are consistent if the URLs are consistent.
This is trying to log this first error case, right?

I think it should be fatal if the user configures this wrong for the initial-cluster.
https://github.com/coreos/etcd/blob/master/Documentation/0.5/clustering.md#error-cases