Supporting both Python 2 and Python 3 with six

We should use six.text_type for both python2 and python3 for
textual data type, not unicode, it supports python 2 only.

Change-Id: Ia5d7b5df83ff1a7536954f6a190502f117c80ff2
Partial-Bug: #1249886
Merge "Supporting both Python 2 and Python 3 with six"