Replace .iteritems() with six.iteritems() everywhere except in
module_utils (because there's no 'six' on the remote host). And except
in lib/ansible/galaxy/data/metadata_template.j2, because I'm not sure
six is available there.
FieldAttributes will now by default not be post_validated unless a flag
is set on them in the class, as a large number of fields are really there
simply to be inherited by Task/PlayContext and shouldn't be templated too
early.
The other (unrelated to the base issue) in #12084 is also fixed here, where
the roles field is loaded before vars/vars_files, meaning there are no vars
yet loaded in the play when the templating occurs.
Fixes#12084
