git-trace logging for gitlab-shell

Instead of using a custom folder for this logging as Yorick suggested, I've decided to just use other file in the same service forlder than gitlab-shell. In this way logrotate is going to work just with the current configuration. Because it used a concept services (which include the gitlab-shell service and its log directory) and rotate all the *.log files in that folder (as denoted by the default logrotate template).

Relates with https://gitlab.com/gitlab-com/infrastructure/issues/619