Limit the amount of prints during normal runs

This removes some Debugf() calls and chages some direct prints to
Debugf(). This means we don't get a bunch of spew when running the
tests.
Allow specifying the docker client path in _DOCKER_CLIENT_PATH

I currently need this to get the tests running, otherwise it will
mount the docker.test binary inside the containers, which doesn't
work due to the libdevmapper.so dependency.