Prepare matrix-registation for (SQLite + Postgres) support

Auto-migration and everything seems to work. It's just that
matrix-registration cannot load the Python modules required
for talking to a Postgres database.

Tracked here: https://github.com/ZerataX/matrix-registration/issues/44

Until this gets fixed, we'll continue default to 'sqlite'.

roles/matrix-registration/tasks/validate_config.yml

@@ -18,3 +18,10 @@
   when: "item.old in vars"
   with_items:
     - {'old': 'matrix_registration_docker_repo', 'new': 'matrix_registration_container_image_self_build_repo'}
+
+- name: Fail if Postgres usage attempted
+  fail:
+    msg: >
+      matrix-registration doesn't support using Postgres just yet.
+      Learn more here: https://github.com/ZerataX/matrix-registration/issues/44
+  when: "matrix_registration_database_engine == 'postgres'"