Spawn RADVD only in the master HA router

Spawn RADVD only in the master HA router

Currently radvd is spawned in all the HA routers irrespective of the
state of the router. This approach has the following issues.

1. While processing the internal router ports (i.e., qr-xxx), ha_router
   removes the LLA of the interface and adds it as a VIP to Keepalived conf.
   Radvd daemon is spawned after this operation in the router namespace
   (if the port is associated with any IPv6 subnets). Radvd notices that
   qr-xxx interface does not have the LLA, so does not transmit any Router
   Advts. In this state, VMs fail to acquire IPv6 addresses because of the
   missing RAs. Radvd does not recover even after keepalived configures the
   LLA of the interface. The only solution is to restart/reload radvd daemon.
   Currently keepalived-state-change monitor does not do any radvd related
   operations when a state transition happens. So we endup in this state
   forever.
2. For all the routers in Backup state, qr-xxx interface does not have LLA
   as it is managed by keepalived and configured only on the Master HA router.
   In such agents syslog is flooded with the messages [1] and this can cause
   loss of other useful info.
   [1] - resetting ipv6-allrouters membership on qr-2e373555-97

This patch implements the following.
1. If the router is already in the Master state, we configure the LLA as a VIP
   in keepalived conf but do not delete the LLA of the internal interface.
2. We spawn radvd only if the router is in the Master State.
3. Keepalived-state-change monitor takes care of enabling/disabling radvd upon
   state transitions.

Closes-Bug: #1440699
Change-Id: I351c71d058170265bbb8b56e1f7a3430bd8828d5
(cherry picked from commit 2f9b0ce940099bcc82d2940b99bdc387db22d6fc)