RHEL 7 support

[danfoster]

This PR addresses some problems to fix some issues RHEL 7 support.

1. Use $releasever instead of the hardcoded 5 in the baseurl for the yumrepo.
2. Don't force the upstart service type if it's a redhat family machine. Let puppet infer that it's systemd.
3. Because the chef installer symlinks the systemd service to /etc/systemd/system/default.target.wants/gitlab-runsvdir.service, it is not enableable. But it gets started on boot due to it being under default.target.wants. The work-around is just to tell puppet not to try and enable the service.

For info, you get the following error in you try to enable the service.

# systemctl enable gitlab-runsvdir
Failed to issue method call: No such file or directory

[tobru]

Thanks a lot for this PR