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.
Ah, so there's a sane default for each that we can proceed with? I'm okay with that (ie, the final choice is not fatal)

I take it it'd be for the non-discovery, non-initial-bootstrap case? IE, a node joining a cluster? Then that's cool, and deserves checking herein.