JenkinsX: Build, test and deploy Nuxeo core server in multi-branches and with PR

The goal is to be able to set up a build for https://github.com/nuxeo/nuxeo so that:

• we can run all junits against an external database ( Postgresql or Mongo)
• we can have a preview env for a pull request with a custom deployment ( ES + Mongo for example)

Check how we handle feature branches: multibranch pipeline?
Discuss with the core team to make sure we cover all their use cases.