pcn-k8sfilter: use init-only-config for ports' parameter

after 18becf4 ("polycubed: fix list validation") a parameter that can only
be set at initialization time must be "config true" and use the init-only-config
extension of polycube.

Fixes: ab259a9 ("Ported k8sfilter")

Signed-off-by: Mauricio Vasquez B <mauriciovasquezbernal@gmail.com>

Author: mauriciovasquezbernal

src/services/pcn-k8sfilter/datamodel/k8sfilter.yang

@@ -22,7 +22,8 @@ module k8sfilter {
           enum INTERNAL { description "Connected to LBRP"; }
         }
         mandatory true;
-        config false;
+        config true;
+        polycube-base:init-only-config;
         description "...";
       }
     }