Use IP addresses in HAproxy

This release moves to using IP addresses whenever possible in HAproxy. Sidecar also now carries IP address information in the gossip messages. If the IP address is specified on a port in Docker, it will be carried in the gossip message and used in HAproxy. If the port is bound on 0.0.0.0, then the public address of the host is used in the gossip messages and HAproxy. A configuration option of use_hostnames has been added to the sidecar.toml to revert to the previous behavior, or for scenarios where that behavior is just more desirable (e.g. NAT situations).