eliminate dependency on ethtool and conntrack on host

We package ethtool and conntrack into a `weavetools` image and run that as a short-lived container with `--privileged` and `--net=host` (or, in one case `--net=container:<id>`).

We use a plain ubuntu container as a build environment, so a) we don't require onerous tooling on the build host, and b) don't pollute the build host with build artefacts. The latter arise because building conntrack requires 'global' installation of its dependencies.

Closes #101.