Add testing jobs for cross-version compatibility

*Original comment by @andrewvc:*

We're working on formalizing x-pack version compatibility and need to expand our test matrix before we do to ensure that the versions we think work together actually do work correctly together. The authoritative list is a WIP. at the moment.

We need additional Jenkins jobs to test that matrix. Probably one parent job and a bunch of child jobs. These should be run daily most likely, as any other schedule will probably be too slow. We shouldn't decide this however until the jobs are built.