vrrpd, yang: cleanup vrrp nb conversion

vrrpd, yang: cleanup vrrp nb conversion

- Use correct units and conversions in model & code
- Fix incorrect CLI help string for V6 virtual addrs
- Fix nb get-entry callback for virtual router
- Fix a couple style nits
- Simplify some CLI code
- Remove unused code
- Remove unused YANG definitions
- Update sighup() to handle reloads
- Update interface level config writer to use NB callbacks
- Add simplified `no` forms for priority and advertisement-interval
  commands

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