Allow JCentral address override using environment variables
This is now needed for the JCentral replica, and we have similar feature in alien.py (
We used to do this by patching the jbox conf files in /tmp, but environment variables are easer to use in CI.
The variable used here is
JALIEN_HOST (as in the jclient file) and default value is of course
We should synchronise all those environment variables at some point.