Migrate Python 2.7 storage sample to Python 3 (#2989)

* Initial commit for storage sample

* Added Python 3.7 version

* Starting urlfetch samples

* requests samples work in 2.7 and 3.7 (gcloud deploy app app3.yaml for the Python 3.7 version)

* urlfetch async sample Working in Python 3 App Engine

* Working in Python 2 App Engine, too

* Added storage test for migration

* Use standard environment variable name for bucket

* Added tests

* Separate out the storage samples

* Remove old debugging line.

Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>

Author: engelke