postgres: Allow installing a specific version

postgres/tasks/main.yml

@@ -3,7 +3,7 @@
 - name: ensure postgres packages are installed
   apt:
     name:
-      - postgresql
+      - postgresql{{ "-{}".format(postgres_version) if postgres_version is defined }}
       - python-psycopg2
       - python3-psycopg2
       - libpq-dev