Replace unicode with six.text_type

Replace unicode with six.text_type

The "unicode" type was renamed to "str" in Python 3. Use six.text_type
to make Cinder compatible with Python 3.

The initial patch was generated by the unicode operation of sixer tool:
https://pypi.python.org/pypi/sixer

Manual changes:

* cinder/volume/drivers/san/hp/hp_3par_iscsi.py:
  replace "isinstance(value, str) or isinstance(value, unicode)"
  with "isinstance(value, six.string_types)"
* cinder/volume/drivers/san/hp/hp_3par_iscsi.py:
  bump the version to 2.0.17
* Revert changes on hacking: hacking explicitly searchs the "unicode"
  pattern for Python 2. Hacking may require more work to be ported to
  Python 3.
* Revert changes on docstrings and strings

Change-Id: I68ea8a81a66c9377b2fe20e3f545dce8b691c398