Remove default values for pki_ca_signing_*_path
    
    In the future pkispawn will validate all path params so the
    default values for pki_ca_signing_csr_path and
    pki_ca_signing_cert_path need to be removed since they point
    to non-existent files. When the params are actually used for
    installing an external CA, CAInstance.__spawn_instance()
    will initialize them with the correct paths.
    
    Signed-off-by: Endi Sukma Dewata <edewata@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>