fix(services): use COREOS_PUBLIC_IPV4 for advertised IP

Currently, all containers use a hacky method to determine the IP
to advertise to etcd - the method determines the machine's default
route, and uses that IP. However, this can be configured very differently
depending on the environment. Instead, we use COREOS_PUBLIC_IPV4, as that
is set on every CoreOS host, and we trust that CoreOS knows the machine's
networks better than we do.

fixes #845