Make runner memory configurable

roles/jupyter/tasks/jupyter.yml

@@ -111,7 +111,7 @@
           'tmpfs_mode': 0o1777,
         },
       ]
-      c.Spawner.mem_limit = '16G'
+      c.Spawner.mem_limit = '{{ jupyterhub_runner_mem_limit | default('16G') }}'
       # Default to the Notebook UI.
       c.Spawner.default_url = '/tree/'
       # Authentication.