Upload from playground

When running tests jobs from playground it's often useful to upload the artifacts somewhere to:

• check that the upload codepath works
• check that any new artifact is published correctly
• have some place to download the artifacts to be tested

Non-production runs end up under https://images.apertis.org/test/${JOBNAME}, such as https://images.apertis.org/test/playground/em/apertis-18.09/images/debos-image-build/ for the playground/em/apertis-18.09/images/debos-image-build job.

This merge request includes:

• ddc16025 Jenkinsfile: Push ostree repositories from test jobs
• a4d7461e Jenkinsfile: Upload even from test job
• dd9f652e Jenkinsfile: Create upload folders if they do not exist
• d619fc62 Jenkinsfile: Explicitly check if remote OSTree branch exists