Fix dynamic groups definition when using --limit (#11577)

Using the hosts directive at the play level prevent those tasks from
being run when using --limit and the group in question is not part of
the limit (ex: running scale.yml on new worker nodes only)

Instead, run on all hosts, and for each group, partition between that
group and '_' (generic group name which is not used; using an empty
string as the group is not supported by ansible.builtin.group_by)

Reported-by: asteppat <asteppat@cisco.com>