/etc/default/gitlab must be sourced before paths are built

The /etc/default/gitlab file is sourced after the various paths are built (pid_path, socket_path, ...), which causes those paths to always use the default `/home/git/gitlab` directory.

This PR fixes this by sourcing the file directly after app_git and app_root are set. A documentation bug (which states where the example gitlab defaults file can be found), is also fixed.