yang: add v2 support to vrrp yang model

yang: add v2 support to vrrp yang model

This adds support for VRRPv2 to the VRRP YANG model, which entails the
following changes:

- Add a version leaf with a default of 3
- Split v4 and v6 vritual addresses into separate lists, with
  constraints on v6 addresses to only exist when the version is 3

General improvements:

- Remove unused config groupings
- Add administrative shutdown leaf
- Split state reporting into v4 and v6 blocks, since these are
  considered separate virtual routers

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>