IPv6 SLAAC subnet create should update ports on net

IPv6 SLAAC subnet create should update ports on net

If ports are first created on a network, and then an IPv6 SLAAC
or DHCPv6-stateless subnet is created on that network, then the
ports created prior to the subnet create are not getting
automatically updated (associated) with addresses for the
SLAAC/DHCPv6-stateless subnet, as required.

Change-Id: I88d04a13ce5b8ed4c88eac734e589e8a90e986a0
Closes-Bug: 1427474
Closes-Bug: 1441382
Closes-Bug: 1440183
(cherry picked from commit bd1044ba0e9d7d0f4752c891ac340b115f0019c4)