Improve REST API test Jenkins X pipeline

Following NXP-27196, let's think about/try to:

• Check if we want the default Git commit status set by Jenkins (e.g. continuous-integration/jenkins/pr-head — This commit cannot be built?
• Should we use a lightweight container instead of the "nodejs" one to install the nuxeo helm chart, as we don't need node bit only access to the helm/jx/kubectl commands?
• Also run the tests against Nuxeo configured with MongoDB in parallel.
• Leverage the PR labels to decide against which backend we want to run the tests, and maybe some other configuration?
• Handle a timeout for the kubectl rollout status deployment command: use njx k8s rollout or do it in bash?
• Handle Slack/email notifications.