Reintroduce 0x for master keys
    
    In cfad6f5bc5be56a22c2110a45645b7cfb49e423b the 0x prefix was
    removed from PGP key IDs referencing Salt Master keys to prevent
    import_gpg_keys.sh from fetching these keys from keyservers, which
    inevitably fails.
    This however introduced a regression with encrypt_pillar.sh, which
    relies on the 0x suffix to find recipients to encrypt against.
    To solve this, the 0x suffix is reintroduced, and import_gpg_keys.sh
    is adapted to instead check for already imported keys - Salt Master
    keys are imported from local files before the keyserver loop, making
    them present as long as the files exist in the repository.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>