Force use of yoga upper constraints in ipa-builder

IPA itself is still compatible with Python 3.6, but ipa-builder uses
master upper-constraints which have dropped support for 3.6 and are
pulling importlib-metadata===4.11.4, which requires 3.7.

    ERROR: Cannot install ironic-python-agent==8.6.1.dev13 because these package versions have conflicting dependencies.

    The conflict is caused by:
        ironic-python-agent 8.6.1.dev13 depends on importlib_metadata>=1.7.0
        The user requested (constraint) importlib-metadata===4.11.4

Change-Id: I0cc48d0d5ed17400badb081e9117c9351677bb38

ansible/group_vars/all/ipa

@@ -47,7 +47,8 @@ ipa_build_dib_env_default:
   DIB_RELEASE: "8-stream"
   DIB_REPOLOCATION_ironic_python_agent: "{{ ipa_build_source_url }}"
   DIB_REPOREF_ironic_python_agent: "{{ ipa_build_source_version }}"
-  DIB_REPOREF_requirements: "{{ openstack_branch }}"
+  # TODO(priteau): Revert once we use c9s.
+  DIB_REPOREF_requirements: "stable/yoga"
 
 # Dictionary of additional environment variables to provide to Diskimage
 # Builder (DIB) during IPA image build.