security: add gpg command to display fingerprints of downloaded keys
    
    Users who wish to verify the Fedora GPG keys may not yet be familiar
    with the gpg command and it's myriad options.  Provide the command to
    display the fingerprints of the downloaded key ring.
    
    Note that gpg will display the fingerprint by default, but without any
    spaces.  That doesn't match what we display, nor what the dnf command
    displays.
    
    Using `gpg --with-fingerprint` adds spaces to the fingerprint display.
    However, there is a subtle discrepancy between the way we (and dnf)
    format the fingerprints and gpg displays them.  In the gpg output there
    are 2 spaces between the fifth and sixth chunks.
    
    Since it's unlikely we're going to get dnf to change its output, we
    might as well match dnf rather than gpg. :)