gitlab: introduce a check for validate DCO sign-off

gitlab: introduce a check for validate DCO sign-off

This introduces a CI job for validating DCO sign-off in every commit
message. The CI jobs are not provided any information on what the
baseline commit for the branch was. We can't compare against the forked
repo's master branch, as there's no guarantee the user is keeping master
up2date in their fork. Thus we add the master upstream repo as a git
remote and identify the common ancestor.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>