Fix Python 3 issues in backup

Fix Python 3 issues in backup

* Replace (int, long) with six.integer_types: the "long" type has been
  removed in Python 3
* Replace str.encode("bas64") with base64.encodestring(str), base64
  text codec has been removed in Python 3. Same change for decode
  (base64.decodestring)
* On Python 3, encode JSON to UTF-8
* tox.ini: add the following tests to Python 3

  - cinder.tests.unit.test_backup
  - cinder.tests.unit.test_backup_driver_base

Blueprint cinder-python3
Change-Id: I86e04f8fbe9a3ce8849fd141dc3ee914e73c8796