PR #1217 mistakenly updated the Set logic when flattening backends,
which caused some cascading errors and wasn't strictly necessary to
resolve the issue at hand. This backs out those changes, and instead
makes the smallest possible change to resolve the initial error, by
separating the logic for flattening regional backends from the logic for
flattening global backends.
We had several calls to `d.Set` that returned errors we weren't
catching, that turning on the panic flag for the tests caught. This PR
addresses them, and fixes the one test that was not safe to run in
parallel because it relied on a hardcoded name being unique.
This is largely just removing calls to `d.Set` for fields that don't
exist in the Schema, fixing how Sets get set, correcting typos, and
converting types.
* Used standard validation functions where possible, added a GCP name validation function.
* Add tests for GCP name, factor out a ValidateRegexp function.
* make fmt
