Dont start if path is nonexistent

This should handle #272 (closed) .

Even when /opt is mounted as a separate filesystem, systemd will attempt starting GitLab and if for some reason all filesystems are not mounted this will make GitLab start fail.

Fix contains a move of the service file to the installed packages unit location of /usr/lib/systemd/system and also adds a condition to start GitLab only after all system services are loaded.