Load librpm on demand for IPAVersion
    
    ctypes.util.find_library() is costly and slows down startup of ipa CLI.
    ipaplatform.redhat.tasks now defers loading of librpm until its needed.
    CFFI has been replaced with ctypes, too.
    
    See: https://pagure.io/freeipa/issue/6851
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>