etcdserver: not get cluster info from self peer urls

Self peer urls have not started to serve at the time that it tries to
get cluster info, so it is useless to get cluster info from self peer
urls.

fixes https://travis-ci.org/coreos/etcd/builds/40354752 in #1664 
if g := getOtherPeerURLs(cl, tt.self); !relect.DeepEqual...