Migrate Istio to use Config Merger

[chases2]

testgrid.k8s.io is now using Config Merger. We should use that instead of Transfigure to put our results on TestGrid, so we don't have to be gated by PR approvals.

TO-DO list:

• Add your Configurator jobs (like this example) to your Prow instance.
• In k8s/test-infra, add your instance to the mergelists and delete the gen-config.yaml file that Transfigure has been maintaining.
• Delete any leftover Transfigure jobs and close any leftover Transfigure PRs.

/assign

[chases2]

Current Transfigure PR: kubernetes/test-infra#23436

[chases2]

New jobs are looking good: #3535

[chases2]

Manually restarted the config merger (known issue GoogleCloudPlatform/testgrid#517) and Istio's config is merged onto TestGrid.