stack/slapos: Version up
slapos.core to 1.5.6 and slapos.toolbox to 0.104: * cleanup stale monitor files and stale promises * support download-from-binary-cache-force-url-list option /reviewed-on !679
slapos.core to 1.5.6 and slapos.toolbox to 0.104: * cleanup stale monitor files and stale promises * support download-from-binary-cache-force-url-list option /reviewed-on !679