tcomp for cloud : make service start script understand environment vars for nexus setup

The tcomp service may use a remote maven repository and today the configuration is done using a settings.xml fiel located in the config folder.
But for the docker image we need to get the repo url as well as login/password as environment variable that can be set by the provisioning process when launching the image.
We will use the following environment variables
MVN_REPO_URL
MVN_REPO_LOGIN
MVN_REPO_PASSWORD