NETVIRT-1080 : Fix PNF discovery failure after FIP detached

NETVIRT-1080 : Fix PNF discovery failure after FIP detached

Problem:
  In conntrack NAT mode, deleting last FIP port on dpn also
  deleting the PNF flow entries ion the OVS node.
Solution:
  The problem is caused by missing ExternalFixedIP entry
  in the vpn-to-dpn-list for the external subnet on the
  NAPT switch. The existence of this entry will prevent
  deleting the PNF flows when the FIP is disassociated.
  The patch fix the problem by adding/deleting the
  ExternalFixedIP entry when the external subnet is
  associated/disassociated with the router.

Change-Id: Ib46e860448138c6808105573ae4c52be0bc2b437
Signed-off-by: Vinh Nguyen <vinh.nguyen@hcl.com>