Adds the scaffolding required for tests to use boto3 and converts the
test_bucket.py tests to the new interface. Follow on patches will
convert the other tests to use the boto3 library.
Notable changes: we no longer try to reach for the equivalent of
`boto.make_request()` and instead rely on the boto3/botocore event
system to mutate requests as necessary (or to disable pre-flight
validators).
Partial-Bug: 1557260
Change-Id: I3d77ef4a6b878c49ebfa0c8b8647d7199d87601e
d4e7940457