Configure gw_iface for RAs only in Master HA Router

Configure gw_iface for RAs only in Master HA Router

For an HA Router which does not have any IPv6 subnets in the external network
and when ipv6_gateway is not set, Neutron configures the gateway interface of
the router to receive Router Advts for default route. In an HA router, only
the Master instance has the IP addresses while the Backup instance does not
have any addresses (including LLA). In Kernel version 3.10, when the last
IPv6 address is removed from the interface, IPv6 proc entries corresponding
to the iface are also deleted. This is however reverted in the later versions
of kernel code.

This patch addresses this issue by configuring the proc entry only for the
Master HA Router instance instead of doing it un-conditionally.

Closes-Bug: #1494336
Change-Id: Ibf8e0ff64cda00314f8fa649ef5019c95c2d6004