keys from nicknym and other domain than it's own should be of 'Weak Chain' validation level

nicknym server is athoritative for it's own domain, but for others it might retrieve keys from key servers. We'll tag keys as 'Weak Chain' until we have more info about the source of the key.

(from redmine: created on 2015-03-30, closed on 2015-04-07, relates #6718 (closed))