Remove non-existent default pki_cert_chain_path
    
    In the future pkispawn will validate all path params so the
    default value for pki_cert_chain_path needs to be removed
    since it points to a non-existent file. When the param is
    actually used (e.g. for installing with an external CA)
    CAInstance.__spawn_instance() will configure the param to
    point to the actual cert chain.
    
    Signed-off-by: Endi Sukma Dewata <edewata@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>