Don't force unicode strings for UUID coercion

Change Ic6b6308fb1960ec40407e6efde30137b64543e72 attempts to
fix difference between Python 2 and Python 3 values by switching
from using str() to formatting into a unicode string (u"%s").

This is equivalent to changing str() to unicode(), but that is
not correct for expected default string types for Python 2.
This requires either using six.text_type(), or just formatting
into a string, without forcing unicode ("%s"), to be correct on
either runtime.

Change-Id: I178f14cdc670d7a696778891e587ef75de208fc2
Closes-bug: #1763179
(cherry picked from commit b719764b)