defaults: Raise descriptive error

Change-Id: I911cb6813f71c8a8a3976d323d1bc1a144bc7445
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
defaults: Add IPv6 class

A class is added to handle IPv6 related configuration keys.

Change-Id: I7f21e914ad1ccd1e0d9c49cf67ae1e87aa094274
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>