py3.x: Use six.text_type() instead of unicode()

Python 3 doesn't support unicode(), use six.text_type instead.
Partial-Bug: #1284677
Partially-implements blueprint py3-compatibility

Change-Id: I5f66ce70e838fedc955f5284c3c480bc37420107
Merge "py3.x: Use six.text_type() instead of unicode()"