[CI] Design/implement flaky test testing for pipelines

[brianseeders]

Create and document a process to replace the flaky test testing method from matrix jobs (editing jobs.yml and duplicating the same ciGroup over and over).

Possibilities

• Create a separate job driven by a separate Jenkinsfile
  • Move shared functionality between the Jenkinsfiles to a separate groovy file
  • Job can be triggered by a comment in a PR, then no editing of files in PR is required
  • Separate CI status will make sure that PRs don't get merged until they're tested by the default job too

[brianseeders]

I ended up reverting the commit for this, because the master job was failing in CI and I need to look into it more.

https://kibana-ci.elastic.co/job/elastic+kibana+master/391/console

There's a problem with how the shared library in the Kibana repo is getting loaded. The fetch command shows that its timing out after 10 minutes, which doesn't happen when running the same job on my sandbox Jenkins.

[brianseeders]

Initial implementation has been rolled out https://kibana-ci.elastic.co/job/kibana+flaky-test-suite-runner/