Change internal rsa_(public|private)_key variable names
    
    In two places the vault plugin refers to rsa public or rsa private key
    although the code can handle just any kind of asymmetric algorithms,
    e.g. ECDSA. The patch just renames the occurences to avoid more
    confusion in the future.
    
    Reviewed-By: Simo Sorce <ssorce@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>