* Replace base64.b64encode() with
oslo_serialization.base64.encode_as_text() which accepts bytes and
text strings but only outputs text string.
* Use a text string for the JSON payload
* Remove test_validators from tests-py3-blacklist to run it
Partially implements: blueprint barbican-py3
Change-Id: Id97104ca9fb0d2acd6e72e6254ca985237e27859