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 still need to sha the initial name. Or the same url list always generates the same nodeID. There is no way for a user to change nodeID without changing url.

We encourage user to use different (nodeName, urls) combination to set up a node if the data of the old node got lost. 
Basically, we want unique nodeID for each node.