Peer configuration via ETCDCTL_PEERS env variable

Add an environment variable for configuring peers.  We have many clusters with individual etcd clusters and we can set this environment variable on each machines in their respective clusters, then we can just run `etcdctl .. ` commands without specifying `--peers` each time.

This was the obvious way to implement it, but could be done another way if there are better ways (default in `etcdctl.go`, maybe?).