add submodule 'gecos-buildtools' and delegate build and test jobs to jenkins

Gitlab CI currently does not provide a way to keep track of test results history.

Jenkins on the other hand do a great job at this, however it currently does not provide a convenient way to add manual deploy jobs:

• build promotion is not supported for pipeline jobs (https://issues.jenkins-ci.org/browse/JENKINS-36089)
• using 'input' (with global 'agent none') puts the build on hold instead of considering the step as optional..

Thus, CI Build and Test are now delegated to Jenkins:

• The Gitlab 'build' job triggers the corresponding jenkins job
• wait until it finishes
• download its updatesite artifact and archive it

The deploy jobs are still handled by Gitlab CI.

WARNING: (re)running the pipeline of an old commit does not work! As a workaround, you need to replay the Jenkins build for that commit (so that it becomes the latest build of the Jenkins job) and then rerun the Gitlab CI pipeline.