Replace dict.iteritems() with six.iteritems(dict)

This change is needed to make Ceilometer code both Python 2.x and
3.x compatible in some future. Oslo modules are left without
changes in this change, that will be done directly in the
oslo-incubator code.

Partially-Implements: blueprint ceilometer-py33-support
Change-Id: Ic7fe5819f85524bb36f04dec6cad10d4cf2b935d
Merge "Replace dict.iteritems() with six.iteritems(dict)"