refactor with_container_netns
refactor with_container_netns

make it set CONTAINER_NETNS instead of CONTAINER_PID (well, it still
sets the latter, but not for effect), which reduces coupling.

n054/weave,dpw/weave,weaveworks/weave,dpw/weave,weaveworks/weave,n054/weave,dpw/weave,paulbellamy/weave,weaveworks/weave,dpw/weave,n054/weave,n054/weave,paulbellamy/weave,paulbellamy/weave,paulbellamy/weave,n054/weave,weaveworks/weave,dpw/weave