Introduce RepositoryKeyFetch=

Introduce RepositoryKeyFetch=

This setting controls whether we'll fetch GPG keys remotely or not.
We disable it by default so that we only rely on locally available GPG
keys for checking package and repository metadata signatures.

This new setting only affects dnf/zypper based distributions as apt
and pacman do not support retrieving GPG keys remotely in the first
place.

zypper does not trust GPG keys listed in gpgkey= by default so we import
local GPG keys manually with rpm to work around that.

Fixes #757