Support for jobs using the cloudbees folder plugins

This patch allows to generate XML of job definitions that are using
the cloudbees' Folder plugin (instead of generating an error):

// will create build/jobDsl/generated/folder_test.xml
folder {
 name 'folder_test'
}

// will create build/jobDsl/generated/folder_test/my_job.xml
job {
 name 'folder_test/my_job'
 ...
}

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>