Migrate our jenkins based CI to gitlab-ci

This is just an issue to track our progress and goals on moving from jenkins to gitlab-ci.

Goals:

• Have at least the exact same CI level as what we have on Jenkins:
  • Run on last stable release branch
  • Build on windows/mac OS/linux (ubuntu+fedora)/IOS/android
• Run all CI on each and every MR before merging
  • What "level" of CI do we want on MRs? (ie. should we run validate tests? What cerbero configs should we test, etc...)
  • How do we run CI on MR with interdependencies (eg. a MR in -bad requires a commit from -base)? We need to make sure that the end result is properly reported on MRs for each component.
• Run CI on forks so that developer know that it passes before proposing a MR (same issue about interdependencies between GStreamer components)
• Allow any developer to update the various component of our CI
• Properly report test results in a way that allows us to follow results history

(Those are the ones I have on the top of my head and it should be updated/better defined).

As discussed several times at hackfest, we should have Docker images to run our CI against, Edward started that, and I buily a little .gitlab-ci.yml file to get those automatically updated with gitlab-ci: https://gitlab.freedesktop.org/thiblahute/gst-ci/pipelines. This might be a good starting point to build upon for linux at least.

Afaik there is no Mac or Windows gitlab-runner on gitlab.fdo yet, that should be added.

Also I started a little .gitlab-ci.yml for gst-build: https://gitlab.freedesktop.org/thiblahute/gst-build/ which has 3 stages, one to build a "manifest", one that builds (currently only in the arch container) and a last one that runs check+validate tests and exports the results using allure. This is me playing with the idea, but I have idea about how to fulfill previously described goals based on what I have and we should discuss it I think.