jenkins-job-builder recursive job groups

[Khai Do]

It would be nice if a JJB job-group could wrap multiple job-groups. This would possibly cut down on a lot of redundancy when calling the jobs.

Example:

- job-template:
    name: jobA

- job-template:
    name: jobB

- job-template:
    name: jobC

- job-group:
    name: groupZ
    jobs:
         - jobA
         - jobB

- job-group:
    name: groupY
    jobs:
         - jobC

- job-group:
    name: groupX
    jobs:
         - groupZ
         - groupY