Rename playbook variables so they are consistently prefixed

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

roles/matrix-server/templates/usr-local-bin/matrix-postgres-cli.j2

@@ -5,5 +5,5 @@ docker run \
 	--rm \
 	--env-file={{ matrix_environment_variables_data_path }}/env-postgres-pgsql-docker \
 	--network {{ matrix_docker_network }} \
-	{{ matrix_docker_image_postgres_to_use }} \
+	{{ matrix_postgres_docker_image_to_use }} \
 	psql -h {{ matrix_postgres_connection_hostname }}