etcdserver: Calculate IDs based on PeerURLs and --initial-cluster-name

Removes the notion of node name being anything more than advisory or
command-line grouping, and adds checks for bootstrapping the command
line. IDs are consistent if the URLs and cluster name (as a token) are consistent.

Adds a flag --initial-cluster-name and mentions it in clustering.md
Six of one, half-dozen of another. I'd have to unwrap more things inside the function itself. Could do if you want, but it's probably a push in terms of cleanliness