Allow JCentral address override using environment variables

Nikola Hardi requested to merge nhardi/jalien-root:feature-set-jcentral into master

This is now needed for the JCentral replica, and we have similar feature in (ALIENPY_JCENTRAL). 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.

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