Fix py3k-unsafe code in test_get_properties()

This changes the unit test test_get_properties() so that it is
py3k-safe.

This is a follow-up to a comment in review 107092:
https://review.openstack.org/#/c/107092/8/ironic/tests/drivers/test_ipmitool.py

Change-Id: I3fc1c9962d02516aa09b8c22f5c1f940a2173703
Merge "Fix py3k-unsafe code in test_get_properties()"