replace max_fds with sysctl call

The call to max_fds no longer appears valid.  This approach of calling
sysctl is standard across Linux based systems and would be the more
established way to obtain this value.

Change-Id: I1bdbbde0bd2102847a3ab061969115030ea207a6

roles/rabbitmq_limits/tasks/main.yml

@@ -10,7 +10,7 @@
   shell: >-
     "{{ container_cli }}"
     exec $("{{ container_cli }}" ps -q --filter "name=rabbitmq" | head -1)
-    rabbitmqctl eval 'proplists:get_value(max_fds, erlang:system_info(check_io)).'
+    rabbitmqctl eval 'sysctl -n fs.file-max'
   changed_when: false
 
 - name: Verify the actual limit exceeds the minimal value