Replace it.next() with next(it) for py3 compat

Replace it.next() with next(it) for py3 compat

The Python 2 next() method of iterators was renamed to __next__() on
Python 3. Use the builtin next() function instead which works on Python
2 and Python 3.

This patch was generated by the next operation of the sixer tool:
https://pypi.python.org/pypi/sixer

Manual changes:

* cinder/tests/unit/test_cmd.py: use "tpgs = iter([tpg])" instead of
  using a magic mock for tpgs

Blueprint cinder-python3
Change-Id: I261f8cda231d80910f27b9c1329f1d43ccdd77b6