Port image/glance.py to Python 3

Port image/glance.py to Python 3

* Fix usage of six.reraise(): new_exc is the exception value, not the
  exception type
* Replace __builtin__ with six.moves.builtins.
* test_extracting_v2_boot_properties(): set config.glance_num_retries to
  fix the test on Python 3 (comparison between mock and int now raises
  a TypeError on Python 3).
* TestGlanceSerializer: use dictionaries with only one key to have a
  reliable output even if the hash is randomized (hash randomization is
  now enabled by default in Python 3).
* tox.ini: add cinder.tests.unit.image.test_glance to Python 3.4.

Blueprint cinder-python3
Change-Id: I33cd02e1b0666d7b5999b2fdaf469dc59fff1866