Port ceph driver to Python 3

Port ceph driver to Python 3

* Replace xrange() with range(). cinder/volume/utils.py uses
  "from six.moves import range", so "range" is xrange on Python 2.
* Replace '' with b'' for image content to get a bytes string on
  Python 3.

Note: _transfer_data() of cinder.volume.utils is tested by
test_backup_ceph which is already run on Python 3, but the test only
failed with python3 run with -bb. This bug is now fixed.

Partial-Implements: blueprint cinder-python3
Change-Id: Ib69b9ee4669d3b627747c754b1bda1994f0ed2a5