Allow username of additional Postgres databases to be different

We'll most likely use one that matches the database name, but
it's better to have it configurable.

roles/matrix-postgres/defaults/main.yml

@@ -36,7 +36,9 @@ matrix_postgres_container_postgres_bind_port: ""
 # Example:
 # matrix_postgres_additional_databases:
 #   - name: matrix_appservice_discord
-#     pass: some-password
+#     username: matrix_appservice_discord
+#     pass: some_password
 #   - name: matrix_appservice_slack
-#     pass: some-password
+#     username: matrix_appservice_slack
+#     pass: some_password
 matrix_postgres_additional_databases: []