BGP: Set advertisement interval when triggering IPv6 RAs

BGP: Set advertisement interval when triggering IPv6 RAs

This change extends the earlier change which added the ability in BGP to
trigger IPv6 Router Advertisements when an unnumbered neighbor is configured.
In addition to triggering the RAs, the advertisement interval is also set to
10 seconds. This is needed to handle the scenario where the peer may start
later.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-10896
Reviewed By: CCR-4693
Testing Done: Manual, bgp-min, bgp-smoke