Replace deprecated "decodestring"

decodestring() is a deprecated alias, use decodebytes()
https://docs.python.org/3.1/library/base64.html#base64.decodestring

This patch helps to remove this deprecation alias 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: Ibba785b8f9b27a74344c27dc952a8782f10fa697
1 file changed