add `weave stop-dns`
and in the process disentangle stopping dns from `weave stop`
check that weave container has an eth0
since w/o that we get an obscure error.

The most likely cause for not having an eth0 is that the docker daemon
has been configured to disable container networking.
Use full container names when populating weavedns. Fixes #250
Merge remote-tracking branch 'rade/186_stop_dns'
Merge remote-tracking branch 'rade/189_assert_docker_networking'