Populate DNS when starting the weaveDNS container

This changes `./weave` so that when the weaveDNS container is started, the script looks for all the containers with weave addresses, and tells the weaveDNS about them. This means weaveDNS can be (re)started after other containers and still resolve their names.

I decided it would be simpler to use the existing HTTP endpoint rather than add a bulk-update endpoint, because the latter would require some encoding of the container IDs, hostnames and IPs, and that would be a terrible pain to do in shell script. We might want to revisit that if we ever rewrite `./weave`.