migrate: Add a migration tool to go from etcd v0.4 -> v0.5 data directories

Still a WIP, because actually restarting a cluster off of these converted directories doesn't work yet -- but individually the nodes agree on the data. Looking for comments.
is it dup code from https://github.com/coreos/etcd/pull/1578/files#diff-ee1257113da61f86a16ab701f754644bR148? It is good to generate ID in one place.