Automatic merge from submit-queue (batch tested with PRs 49316, 46117, 49064, 48073, 49323)
Test Ubuntu image using GKE image spec on master
Ref: https://github.com/kubernetes/kubernetes/issues/46891
This PR changes the files referenced in test-infra for running Ubuntu image tests against GKE system spec on master.
The two properties files are shared by the tests against all k8s branches but the `SYSTEM_SPEC_NAME` is only available on master. This should be fine because the tests in the non master branches will just ignore the unknown env variable.
**Release note**:
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/assign @yujuhong
6c5b24b564