Add upstart job definition file for Gitblit

The init.d script does not implement the 'status' sub-command, causing log churn
on antimony. So replace it with a modern and concise upstart job definition.
/etc/init.d/gitblit should be stopped and removed manually.

Change-Id: I3f32dedf15ce4c7cc96f6ed16d03dcfadce1d453