Use Pipeline to refactor iOS deployment jobs

Let's refactor deploy-nuxeo-mobile-ios to:

• Split it into 2 jobs: deploy-nuxeo-mobile-ios-testflight and deploy-nuxeo-mobile-ios-appstore. That will be clearer as the first one should be an automatic job triggered on push every time the master branch is merged into the release-ios branch, and the second one should be launched manually.
• Use Pipeline to experiment it on these quite simple jobs.

Using Groovy will probably simplify statements such as:

• deploy-nuxeo-mobile-ios-testflight must only perform the deployment if the commit message doesn't contain "Release", which is currently done with the Conditional steps plugin.
• deploy-nuxeo-mobile-ios-appstore cannot be launched if deploy-nuxeo-mobile-ios-testflight isn't blue.

Here are some interesting links about Pipeline:

• Job sample using a Pipeline script within the job: https://qa.nuxeo.org/jenkins/job/Private/job/deploy-nuxeo.com-staging/configure
• Pipeline syntax generator: choose a step and it generates the Groovy!
  https://qa.nuxeo.org/jenkins/job/Private/job/deploy-nuxeo.com-staging/pipeline-syntax/
• Global variable reference: https://qa.nuxeo.org/jenkins/job/Private/job/deploy-nuxeo.com-staging/pipeline-syntax/globals
• Job sample using a Jenkinsfile: https://qa.nuxeo.org/jenkins/job/Client/job/nuxeo-automation-php-client-master/configure, Jenkinsfile: https://github.com/nuxeo/nuxeo-automation-php-client/blob/2.0-rewrite/Jenkinsfile

Note about using a Jenkinsfile: the problem right now is that Jenkins will do a first checkout from SCM to get the Jenkinsfile, then the Pipeline script itself will do another checkout of the same repository to perform the job tasks, that's not optimized.