Create a function to download docker content

[kdelee]

We need a function to download published docker content for test_restore.py

This should not depend on the docker client but rather be able to parse whatever metadata that the published repo provides about its structure and form its request based on that.

[jeremy-audet]

If such a function was written for Pulp Smash, it would be OK to use that here in pulp-migrate to solve this issue.