Fix IPv6 default masks for SLAAC

IETF doesn't recommend use of IPv6 networks
with masks not "/64". Recommended subnetting is
3-1-4 - 48 for provider, 16 for subnetting and
64 for network. For example SLAAC will not
work if you use subnets different from /64.
Test with port update will not work until
bug 1364166 is fixed, added attribute to it.

Change-Id: I260c820dbde1f98b569e564c7b5e7e558e431c0d
Merge "Fix IPv6 default masks for SLAAC"