Python3: Replace dict.iteritems with six.iteritems

This also adds a check to nova/hacking/checks.py that
should check dict.iteritems, dict.itervalues and
dict.iterkeys should not be used in the future.

NOTE:
In _dict_from_object() method of test_db_api.py items() method
is called on dict and iteritems() method is called if object is
other than dict. Changed it to directly use obj.items as obj can
either be dict or object of nova.db.sqlalchemy.models.<Class> and
both have items() method.

Partially-Implements: blueprint nova-python3-mitaka

Change-Id: Ib0fe3066199b92e4f013bb15312ada7515fa3d7b

HACKING.rst

@@ -54,6 +54,9 @@ Nova Specific Commandments
 - [N341] contextlib.nested is deprecated
 - [N342] Config options should be in the central location ``nova/conf/``
 - [N343] Check for common double word typos
+- [N344] Python 3: do not use dict.iteritems.
+- [N345] Python 3: do not use dict.iterkeys.
+- [N346] Python 3: do not use dict.itervalues.
 
 Creating Unit Tests
 -------------------