Use oslo_serialization.base64 to follow OpenStack Python3
Use oslo_serialization.base64 to follow OpenStack Python3

This patch replaces python standard base64 library call to
oslo_serialization.base64 to follow OpenStack Python3 porting
standard [1], since the requirements.txt has already defined
oslo.serialization>=1.10.0.

[1] https://wiki.openstack.org/wiki/Python3

Change-Id: I77aaeafb3e1f6167ac062f2670f45dd2ebf642dd

NaohiroTamura/python-ironicclient,openstack/python-ironicclient,openstack/python-ironicclient,NaohiroTamura/python-ironicclient