Replace unicode() for six.text_type

To support Python3, unicode() calls has been replaced by
six.text_type.
Added utils.exception_to_str(): is the proper way to convert
an exception to string, since it manages logic related to
encoding.

Change-Id: I27101390e4f95e5c7690b1b445b7e75b8bcb9a08
Closes-Bug: #1284677
Merge "Replace unicode() for six.text_type"