new mechanism for deciding automatically if JBT & JBDS aggregate jobs should fire

We need a series of jobs for 4.0.x, 4.1.x, and master which will:

• install a comma-separated list of IUs (.feature.groups) from a composite site into a pre-existing Eclipse installation, then
• diff whether the fresh installation differs from a previously cached install manifest (list of features/plugins)
• if the install footprint is different from before, the composite site contains new content and we should fire a downstream job to produce a new aggregate site