RFC4861 allows us to specify the Link MTU using IPv6 RAs.
When advertise_mtu is set in the config, this patch supports
advertising the LinkMTU using Router Advertisements.
Partially Implements: blueprint mtu-selection-and-advertisement
Closes-Bug: #1495444
Change-Id: I50d40cd3b8eabf1899461a80e729d5bd1e727f28