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
@jonboulle @barakmich If we removed a node with peerURL X in cluster A. Then we want to add it back to the clusterA, we need to assign it different NodeID. Our current approach is to generate NodeID with hash(peerURL, clusterName, timestamp).