Port test_db_api to Python 3

Port test_db_api to Python 3

* _dict_from_object(): on dictionaries, call the items() method instead
  of iteritems() method. oslo.db objects have no items() method yet, so
  check the object type to decide which method should be called.
* test_db_api: fix usage of db.quota_reserve(), quotas dictionary values
  must be integers (hard limit of quotas), not Quota instances of
  cinder.db.sqlalchemy.models.
* tox.ini:  add the following tests to Python 3.4

  - cinder.tests.unit.test_db_api
  - cinder.tests.unit.test_quota

Blueprint cinder-python3
Change-Id: I93f5e0f2fe34c9a6c135f34d64ec068c5696032d