Python 3: Use six.moves.range

Python 3: Use six.moves.range

The function `xrange` was renamed to `range` in Python 3.

* Remove `xrange` occurences so that Python 3 tests can pass. Use
  `six.moves.range` instead to get the right function in both cases.
* Generalize the use of the efficient `range` (ex-`xrange`) in
  critical sections (when iterating over large lists).
* Simplify code.
* Add a hacking check to prevent future usage of `xrange`.

Change-Id: I080acaaa1d4753619fbbb76dddba6d946d84e73f
Partially implements: blueprint neutron-python3