Add L3 VRRP HA base classes

This patch is a part of the blueprint l3-high-availability

This patch initiates the blueprint by adding base classes
on the plugin side.

A new admin-only ha attribute is added
to the API router resource.
Convertion from or to HA router is possible.

DocImpact

Partially-implements: blueprint l3-high-availability

Change-Id: I9d935cf5e0c231e8cb7af5f61b9a9fc552c3521e