config-generator yaml format doesn't work with i18n fields

This is because there's no yaml representer for i18n Messages object.
This patch aims to add this representer and allow the generation of
configurations using oslo.i18n strings.

One example of this is cinder.

Closes-bug: #1928582
Change-Id: I70ab87c9bed093cad883b6301b8a09753fc470d9