slapos_cloud: Migrate Successor/Predecessor when migrate HS into Instance Tree

   This save us double indexation activities triggerred on different moments.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:20:30

passed SlapOS.Eggs.UnitTest-Master.Python2

00:28:35

passed SlapOS.Eggs.UnitTest-Master.Python3

00:26:09