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.
We don't want to hash the name, as I discussed with @philips and @bcwaldon -- the same URL list creating the same nodeID is intentional, for the static bootstrap case. The cluster is defined (at least at outset) only by the URLs of all the peers, and, in the case of the local node, the URLs it's bound to. 

Adding name (and especially hashing name) makes the whole thing a lot easier to screw up.

Further, and this goes to your second point, we should (and in fact, @unihorn has this patch) store the nodeID in the data directory. If it's truly a lost node starting from scratch, at that point we (a) are already bootstrapped and (b) will hash the time.

TL;DR: names should be purely advisory/metadata and not part of a hash.