Add tests for the cloud object storage

Summary:
Create mock classes for the libcloud driver so the objstorage behavior can be checked.

Also, catch a libcloud exception and raise swh exception instead at the lower possible level in the code.

Reviewers: Reviewers, ardumont

Reviewed By: Reviewers, ardumont

Subscribers: ardumont

Differential Revision: https://forge.softwareheritage.org/D100