Adds a --no-rewrite-hosts flag to the proxy, and be default, removes the
docker IP from hosts. We only discard the docker host when it exists, so
--net=none works.
Due to https://github.com/docker/docker/issues/9098, there is no way to
signal an exec. Instead we'll have to just poll the interface and hope.
Adds `weavewait -s` which is the way to tell weavewait to skip waiting
for a signal, which we use on execs
Paul Bellamy