Use six.iteritems for python 3.3 support in apiclient module

In python 3, dict type does not have iteritems method. So use
six.iteritems for compatibility in apiclient module.

Part of blueprint make-python3-compatible

Change-Id: I5dd0ea8f38f775d89be9b5f8743a273bf2a7520f
Merge "Use six.iteritems for python 3.3 support in apiclient module"