Use a parameter like $download_url instead of $refspec and $ci_job_name

The current version of module uses parameters $refspec and $ci_job_name to magically forge an URL to donwload the JAR file. This can be improved :

• the $refspec is not used by vcs tool, it looks like using a knife like a fork.
• the magic enforce to donwload jar file from gitlab.adullact.net.

Using an URL as module parameter removes the magical fog and permits to download the jar file from others places.