Run external testing in parallel (1 Jenkins job per application)

[smlambert]

Instead of running all the application tests in a single Jenkins job, have a parent job fire off child jobs per app. This is the same approach we use for system tests.

Benefits of this approach are:

• easier to triage (smaller console output and junit output doesn't get garbled with others)
• won't hit the timeout as we add new testing (with recent additions a single job can take up to 10hrs, which is our default timeout on jobs)

[sophia-guo]

Closed by #991