Evaluate matrix/matrix-like job-build via Declarative Pipeline

@davehunt , others:

At some point in the near future, we're going to want to be able to use a Jenkins matrix-build/"multi-configuration project" to auto-generate, better-collate, share configs, etc. between test-run jobs/site URLs -- and do as much as possible using Declarative Pipeline/job-DSL, I think.

In chatting with @abayer today a bit, we might be able to realize build-matrix/multi-config job creation around March 2019; in the mean-time, there's also:

https://support.cloudbees.com/hc/en-us/articles/115000088431-Create-a-Matrix-like-flow-with-Pipeline to consider

https://github.com/mozilla/wpt-api/blob/job-dsl/Jenkinsfile