Just as a hint in case the above fix doesn't work:
Some of the validations rely on plugins that import from the tripleo_validations python package that is installed with openstack-tripleo-validation rpm. AFAICS the rpm isn't installed in any of the mistral containers, so mounting the /usr/share/openstack-tripleo-validations path might not be sufficient.
Just as a hint in case the above fix doesn't work:
Some of the validations rely on plugins that import from the tripleo_validations python package that is installed with openstack- tripleo- validation rpm. AFAICS the rpm isn't installed in any of the mistral containers, so mounting the /usr/share/ openstack- tripleo- validations path might not be sufficient.