When running "tox -e py35", it shows:
DeprecationWarning: decodestring() is a deprecated alias, use decodebytes()
This patch helps to remove this deprecation warning by
replacing "base64.decodestring" with "oslo_serialization.base64.decode_as_bytes".
The same has been done for nova:
https://review.openstack.org/#/c/610401/
Change-Id: Ib3f7fc4392ca53e92ee2e421b95874dc97f04e5f