Tells the proxy not to do anything with this container. We set it when
running weaveexec/weave/etc containers, so that the weave script works
through the proxy.
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
Bug came up in a test where we are restarting and doing an exec checking
a network interface right away. weavewait is only added if the container
has a WEAVE_CIDR var set, the same as at container creation.
Paul Bellamy