ns_server should mark it's admin interface (port 11209) with 'system'

As part of fixing MB-33971 the reserved pool of connections is counting the number of connections to port 11209. Given that this port number is provided by ns_server to memcached (and is a different number when running under cluster_run) ns_server should add "system":true to the JSON description for that port.

ex:

{ 

  "port" : 11209,

   "system" : true,

   "ipv4" : "required",

   "ipv6": "optional",

   "host" : "*",

   "tag" : "ns_server private interface"

}