Enable prometheus jobs + Move caas images to public.ecr.aws/jujuqabot

[hpidcock]

• Enables prometheus job in gating
• Disables test-controllercharm-test-prometheus-aws due to k8s<->machine cmr (easy to do with microk8s+lxd)
• Disables test_pebble_change_updated as it was just dropped from 3.5 until we have a fix for it
• Create public.ecr.aws/jujuqabot/build-${SHORT_GIT_COMMIT}/... repos for each build of juju and reuse that in all k8s tests instead of creating a ecr private repo for each test and rebuilding the image in each test, should improve run times for k8s tests significantly.
• Add support to gen-wire-tool to use the microk8s runner for specific tests on machine cloud tests (since the controller charm/prometheus tests use prometheus-k8s charm).