Add an "IPv6 Impact" section to the template

This change adds a requirement for an "IPv6 Impact" section. In the review for
Kilo priorities [1], the issue was raised around IPv6 being a priority. One
outcome was the thought to ensure new features are IPv6 ready from the start.
This change ensures specs have a section detailing how they affect IPv6.

I've also updated approved specs with what I perceive to be their IPv6
readiness. I hope the original authors can view these to confirm what I've
put in there.

[1] https://review.openstack.org/#/c/136514/

Change-Id: I6b12615d5d650ea55b8e78fd43c702af8f87b3f7