Honor $JENKINS_HOME (just like we used to do $HUDSON_HOME)
We still fall back to $HUDSON_HOME every step of the way
for maximum backward compatibility.

I wasn't initially planning to do this for the first version,
but working on native packages made me think we should deliver this.

For now, I'm leaving /var/lib/hudson and the user/group name intact,
as the users might have been using the 'hudson' user, and
/var/lib/hudson can contain some big data.