Add support for folder jobs
(https://plugins.jenkins.io/cloudbees-folder/) by reworking the service
to support nested jobs.
This also fixes this deprecation warning (as a happy side effect):
WARNING:jenkins_jobs.cli.subcommand.test:(Deprecated) The default output behavior of `jenkins-jobs test` when given the --output flag will change in JJB 3.0. Instead of writing jobs to OUTPUT/jobname; they will be written to OUTPUT/jobname/config.xml. The new behavior can be enabled by the passing `--config-xml` parameter
4bcb22e17a