fix(contrib/ec2): expose etcd to 0.0.0.0

Before #851, we had a hacky way of finding the host's IP address. This PR allows us to use $COREOS_PUBLIC_IPV4 as $HOST in containers and use $HOST:4001 in a component's `/bin/boot` script to find etcd.

This bug surfaced while trying to test #851 on EC2.