Replace dict.iteritems() with dict.items()

Replace dict.iteritems() with dict.items()

This change adds Python 3 compatibility to the modified code.

Replace also six.iteritems(obj) with obj.items().

The iteritems() method of Python 2 dictionaries was renamed to items()
on Python 3. As discussed on the openstack-dev mailing list, iteritems()
must be replaced with items(), six.iteritems() should not be used. In
OpenStack, the overhead of creating a temporary list with dict.items()
on Python 2 is negligible.

Blueprint cinder-python3
Change-Id: Ic3d8fd6b71d2c9f21929b0d6bf68c8f84a5e2567