Reverting timeout for gitlab workers to default suggested value of 30 sec.

I found empirically that my gitlab workers would stick around for longer with the old setting even after stopping the gitlab service, and most importantly, that I would often get 502 proxy errors from Apache, because it wasn't able to reach gitlab. Somehow, using a lower timeout seems to increase the reliability of gitlab.

I used 300 in the past because performing a git clone of a decent size repository over HTTP from a remote computer seemed to bump against the suggested timeout of 30 seconds. I guess I should file a GitLab issue about that.