Accept PGP signature if key exists in Debian keyring, Ubuntu or OpenPGP

Based on existing configuration keys from Debian keyring, Ubuntu key server and OpenPGP key server were accepted. However the Ubuntu key server was only ever used if the OpenPGP server was offline, which never happens.

Make GnuPG fetch keys from both key servers so it is enough that a key is on the Ubuntu key server.

This issue was discovered in !471 (comment 463152)