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
2 jobs for 1.0
Status Job ID Name Coverage
  External
running SlapOS.SlapRunner.ResilienceTest-Master.ERP5

42779:03:24

failed SlapOS.SoftwareReleases.IntegrationTest-Master