Fix subunit describe calls for py3
Fix subunit describe calls for py3

iteritems does not exists in python3, and it's
actually enough to iterate over keys.

Change-Id: I1a614942e25af26fedc720e5ebf8f800e0c54138

vedujoshi/tempest,Juniper/tempest,openstack/tempest,Juniper/tempest,cisco-openstack/tempest,masayukig/tempest,openstack/tempest,cisco-openstack/tempest,masayukig/tempest,vedujoshi/tempest