Simplify keepalived.virtual_routes

Simplify keepalived.virtual_routes

keepalived.virtual_routes previously held one list of virtual
routes of different kinds, and the HA router class manipulated
that list directly. The list held both the default gateway
virtual route, and any extra routes. This means that when adding
extra routes for example, the HA router would first have to
remove all routes that are not default gateway routes, then add
the extra routes received via RPC.

This is messy because:
a) It's needlessly complicated
b) It's fragile
c) There's zero separation of concerns (HA router should not know
   how keepalived maintains its list of virtual routes)
d) It requires changes to the management of the default gateway
   and virtual routes just to add another type of extra routes

This patch solves these issues by separating the persistency of
virtual routes according to their role.

Co-Authored-By: gong yong sheng <gong.yongsheng@99cloud.net>
Related-Bug: 1414640
Change-Id: I1406b1876c3a47b110818686b42e5f2f688154fa