Revert "Remove extra ipv6 conf for Debian."

Hard-coding the ipv6 files for Debian kind of defeats the point of the
network library. However, our network library example42 doesn't provide
any documentation, and no clear examples on how to configure ipv6 for
Debian.

The other alternative, puppet/network, neither provides documentation,
and doesn't even provide any ipv6 examples.

This reverts commit ab786864.