Use -Djava.awt.headless=true on Debian service

I installed the Jenkins Debian package, and later installed an XServer (XVFB).  The graphs of test results etc. were replaced by an image suggesting that I add this flag to the JVM commandline.  I don't see any reason why this shouldn't always be present on Debian systems.
Merge pull request #1087 from thetestpeople/patch-1

Use -Djava.awt.headless=true on Debian service