copr-fe: use lower shared_buffers value on STG

The instance has only 4G of RAM so PostgreSQL fails to start with:

    FATAL: could not map anonymous shared memory: Cannot allocate memory

roles/copr/frontend/tasks/psql_setup.yml

@@ -60,7 +60,7 @@
   lineinfile:
     path: /var/lib/pgsql/data/postgresql.conf
     regexp: '^shared_buffers ='
-    line: 'shared_buffers = 8096MB'
+    line: 'shared_buffers = {% if devel %}1024MB{% else %}8096MB{% endif %}'
   notify: Restart postgresql
   tags:
   - config