yaml-validate.py not compatible with python3

Dear Stackers,

In tripleo-heat-templates, the pep8 tox task fails when we have python3 available.
We can find multiple occurrence of this kind of error:

Validating ./docker/services/octavia/octavia-deployment-config.yaml
Traceback (most recent call last):
  File "./tools/yaml-validate.py", line 1060, in validate
    retval |= validate_service_hiera_interpol(filename, tpl)
  File "./tools/yaml-validate.py", line 897, in validate_service_hiera_interpol
    search_keynames, enter_lists)
  File "./tools/yaml-validate.py", line 858, in _rsearch_keys
    return _rsearch_keys_nested(d, pattern, search_keynames, enter_lists)
  File "./tools/yaml-validate.py", line 813, in _rsearch_keys_nested
    for k, v in d.iteritems():
AttributeError: 'dict' object has no attribute 'iteritems'

For now, a change is proposed in order to fix python2 as the running version¹, but this is a quick fix.

I'm unfortunately not the right one for some python2->python3 migration :(.

Thank you!

Cheers,

C.

¹ https://review.openstack.org/#/c/586518/