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.

Since we've just changed the group ID and break everyone who depend on
the old artifact GAV, might as well rename artifact IDs as well.
(Otherwise we'd break people twice.)

Signed-off-by: NMykola Nikishov <mn@mn.com.ua>