Jobs based on custom runners: docs and gitlab-runner setup playbook

Jobs based on custom runners: docs and gitlab-runner setup playbook

To have the jobs dispatched to custom runners, gitlab-runner must
be installed, active as a service and properly configured.  The
variables file and playbook introduced here should help with those
steps.

The playbook introduced here covers the Linux distributions and
has been primarily tested on OS/machines that the QEMU project
has available to act as runners, namely:

 * Ubuntu 20.04 on aarch64
 * Ubuntu 18.04 on s390x

But, it should work on all other Linux distributions.  Earlier
versions were tested on FreeBSD too, so chances of success are
high.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Willian Rampazzo <willianr@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210630012619.115262-4-crosa@redhat.com>
Message-Id: <20210709143005.1554-4-alex.bennee@linaro.org>