Rename/move/remove HaRouter methods

Rename/move/remove HaRouter methods

All of the methods that use verify_ha are now part of the HaRouter
class. The check has outlived its usefulness and may die
a peaceful yet horrifying death. At its point of death,
verify_ha is akin to a guy walking down the street and yelling:
'Am I me? Am I me?' - Yes, you're you, shut up. An HA router
is an HA router, there's no point in the class checking if
it's indeed an HA router.

* keepalived_manager._get_full_config_file_path was being used
  outside of the keepalived_manager, removed the leading underscore
* _ha_get_existing_cidrs doesn't need an 'ha' prefix, it's already
  a part of the HA router class. This patch renames the method to
  a more descriptive name
* HARouter._get_ipv6_lladdr had nothing to do with HA routers, nor
  did it use the HARouter state. Moved it to ip_lib

Partially-Implements: bp/restructure-l3-agent
Change-Id: I6a7df0b08ca3fc8c2c1b310aa8dd897111f52974