The first call to persisting facts would work due to the assignment of a
MutableMapping calling __setitem__ but subsequent module fact data would
not be propogated to the fact cache plugins because update() doesn't
invoke __setitem__. This changes the behavior a little bit and ensures
set() is called on cache plugins.
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.
