tests/topotests: Pull docker image from registry

tests/topotests: Pull docker image from registry

We have set up automated builds for the topotest images at dockerhub.
Therefore, people don't need to perform a local build to run the
topotests on docker, but can use the image built by dockerhub.

As there is not much benefit in building locally, but the disadvantage
of weird errors if using an outdated image, change the topotests target
to always pull the most recent image from dockerhub.

Add an environment variable to disable this, e.g. for local development.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>