[Feature] Consider verifying checksum of Openstack Storage upload

Currently, it is up to a user to ensure data integrity by sending an `ETag` header during an upload or checking it the response. Is there any interest in moving this checking functionality into `pkgcloud`? Using a through stream we could calculate the checksum during upload and compare at the end.

Questions I still have are:

1. What is the overhead of calculating the checksum? I would guess very small but don't actually know.

1. What happens if the checksum match fails? Probably remove the file and return an error.. but what if the the remove operation fails?