Towards 2.7 - Support Property on Jobs and Folder for Group inclusion


This PR primality adds support for two things 
 * Using a Cloudbees Folder in the same way as a View to select Jobs for a Job group
 * Using JobProperty and FolderProperty to add a Job or Jobs in a Folder to a Job Group.

With this both a "top-down" and a "bottom-up" approaches for creating Job Groups are supported.

@oleg-nenashev and @jglick as I know you know your way around Jenkins and Folders I'd be happy to get some feedback on this from you.

Thanks.

JENKINS-21337	 Add support for cloudbees-folders
JENKINS-21356	 Add support for using JobProperty to join a Job to a JobGroup