Pull images before running inside weaveexec container
Pull images before running inside weaveexec container

Now we don't bundle the docker command inside weaveexec, we need to do
an explicit 'docker pull' to ensure the images are there.

Also set the weavedb tag to :latest to remove a warning from docker

weaveworks/weave,weaveworks/weave,weaveworks/weave,weaveworks/weave