Support multiple IPv6 prefixes on internal router ports for an HA Router

Support multiple IPv6 prefixes on internal router ports for an HA Router

As part of BP multiple IPv6 prefixes, we can have multiple IPv6 prefixes on
router internal ports. Patch, I7d4e8194815e626f1cfa267f77a3f2475fdfa3d1, adds
the necessary support for a legacy router.

For an HA router, instead of configuring the addresses on the router internal
ports we should be updating the keepalived config file and let keepalived
configure the addresses depending on the state of the router.

Following are the observations with the current code for an HA router.
1. IPv6 addresses are configured on the router internal ports (i.e., qr-xxx)
   irrespective of the state of the router. As the same IP is configured on multiple
   ports you will notice dadfailed status on the ports.
2. Keepalived configuration is not updated with the new IPv6 addresses.

This patch addresses the above issues for an HA Router.

Closes-Bug: #1446161
Partially-implements: blueprint multiple-ipv6-prefixes
Change-Id: Icb9a0e4e6e5deafbdc0135ce7e6b100b1725df66