Allow to add router interface to IPv6 SLAAC network

This patch will allow an IPv6 subnet configured with SLAAC
(or dhcpv6-stateless) to be attached to a router interface.

Closes-Bug: #1382076
Change-Id: If0c48a7287a828eef4a0f0b0859d4f898d2937bd
(cherry picked from commit 95accb535017f1384b38b12eb78f0e58287d8e08)