Use Postgres 10.x by default (only for new installs)

This playbook just tries to avoid trying to setup a Postgres 10
database with existing 9.x files, as that makes Postgres complain.

Due to this, existing installs (still on 9.x) are detected
and left on Postgres 9.x.
They need to be upgraded to Postgres 10.x manually.

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

@@ -7,5 +7,5 @@ docker run \
 	{% if not matrix_postgres_use_external %}
 	--link=matrix-postgres:{{ matrix_postgres_connection_hostname }} \
 	{% endif %}
-	{{ docker_postgres_image }} \
+	{{ docker_postgres_image_to_use }} \
 	psql -h {{ matrix_postgres_connection_hostname }}