Docker slave image testing

Next to NXBT-2277 and NXBT-2535, we need an advanced tooling and process.

Use case 1 - branch testing:
The developer want to customize a slave image.
1) the developer customizes the image with a branch on https://github.com/nuxeo/nuxeo-ansible-jenkins
2) his desktop hosts a swarm-slave which will be used to locally build all the image alternatives (no push)
3) the image alternative registers itself (swarm-slave) in Jenkins for being used in the wanted jobs for validating the changes.

Use case 2 - PR staging:
The developer has successfully tested the changes with the above procedure.
1) the developer pushes his working branch as a PR
2 the PR is built and produces images tagged with the PR ref.
3) those images should be usable in the CI as staging/preprod
4) the PR is merged, the images are automatically updated and the staging is deleted.

2.2) automatically or manually?
2.3) how ? the mapping between label and cloud image is hard coded in the global configuration

Notes:

• be careful with the consumed disk space and the cleanup in 2.4)

Related resources:
https://qa.nuxeo.org/jenkins/job/System/job/build-slave-swarm/
https://qa.nuxeo.org/jenkins/job/System/job/build-preprod-slave-images/
https://qa.nuxeo.org/jenkins/job/System/job/build-slave-images/
https://nuxeowiki.atlassian.net/wiki/spaces/NIG/pages/2621594/Docker+Jenkins+slave