gs_utils.py: use boto instead of google-api-python-client library so we can use .boto file credentials

Also adds these public functions:
- delete_file(bucket, path)
- upload_file(source_path, dest_bucket, dest_path)
and a self-test that exercises all functions.

BUG=skia:2732
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/387233003