stack/slapos: version up slapos.core 1.6.18

3 jobs for master
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

passed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

passed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP