Port object_storage tests to Py3.

Main changes:
* Use bytes instead of strings for object content (data)
* Use bytes when using crypto functions (hmac, md5)
* When we can't make the service client to always return strings
(because we can't know for sure the returned payload is a text),
use decode() in tests.

Change-Id: Ic56827e3401f36ccc555dd0568b606a8c3047f00
15 files changed