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
Generation based on NodeID is fine, agreed. But we can't drop the name field from the struct, because we'd like to Set() it -- building nodes with said ID --, and Set() is an interface for flags we can't change