tests: use stubbers for unit tests

[efiop]

• [] s3: http://botocore.readthedocs.io/en/latest/reference/stubber.html (or better to use moto)
• gs: https://cloud.google.com/appengine/docs/standard/python/tools/localunittesting
• http: Use mock instead of HTTPServer #1573

We are already using moto for S3 since #2853

[efiop]

@MrOutis Not really, lots of stuff is still tied to the real aws.

[efiop]

Closing in favor of #4972