Automatic merge from submit-queue
Trusty: Regional release .tar.gz support
@zmerlynn and @roberthbailey please review it. This change is to support the feature added in PR #22234. The entire logic is pretty much the same as in #22234, with only few minor changes in implementation.
I had manually run e2e tests with "export RELEASE_REGION_FALLBACK=true" on two clusters: (1) Trusty on master nodes on ContainerVM; (2) Master and nodes all on trusty. All tests are green. I don't figure out a way to simulate regional fallback. But I did test the function download_or_bust() out-of-box.
cc/ @wonderfly @dchen1107 @fabioy FYI.
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