Fix warning of "Enable ip forwarding" (#6953)

The task outputs the following warning:

  TASK [kubernetes/preinstall : Enable ip forwarding]
  [WARNING]: The value 1 (type int) in a string field was converted
  to u'1' (type string). If this does not look like what you expect,
  quote the entire value to ensure it does not change.

roles/kubernetes/preinstall/tasks/0080-system-configurations.yml

@@ -58,7 +58,7 @@
   sysctl:
     sysctl_file: "{{ sysctl_file_path }}"
     name: net.ipv4.ip_forward
-    value: 1
+    value: "1"
     state: present
     reload: yes