Replace iteritems() to items() in Jinja2 templates (#4437)

The iteritems() dictionary's method has been removed in Python3. Using
this method in Jinja2 templates limits the execution to Python2 which
will be deprecated in 2020[1]. This change replaces that method for
the items() method as it's suggested in the official website[2].

[1] https://pythonclock.org/
[2] https://docs.ansible.com/ansible/latest/user_guide/playbooks_python_version.html#dict-iteritems