HC2VPP-321: bump nat model to ietf-nat@2018-02-23.yang

HC2VPP-321: bump nat model to ietf-nat@2018-02-23.yang

Uses ietf-nat module as defined in
https://tools.ietf.org/html/draft-ietf-opsawg-nat-yang-14

Modifications to original model:
- ip-address is used instead of ip-prefix
 (draft is not clear, see HC2VPP-325).

API changes relevant to HC/VPP:
- nat-config and nat-state merged into nat container
- nat-instances and nat-instace renamed
  to instances and instance respectively
- external-ip-address-pool has new parent - policy list
- port-type choice was removed, port is now defined using
  start-port-number and end-port-number leaves.
  Missing end is equivalent to single-port-number case from
  the older version.

TODOs:
- HC2VPP-322: single nat64Prefix per NAT instance
- HC2VPP-323: update postman collection
- HC2VPP-324: update CSIT tests.

Out of the scope of NAT model update:
- HC2VPP-326: support for nat-type handling
  (vpp-nat model might become obsolete)
- HC2VPP-327: support external-realm container
  (requires adding augmentations
  that are alredy present in the interface-nat model).

Change-Id: Ie229a55b0a3f6ee3f8c97f4fd13c54a797a62438
Signed-off-by: Marek Gradzki <mgradzki@cisco.com>