cli/prune: fix a possible infinite recursion

With recursive instances, it was possible that the slapos.cfg introduce a
kind of loop and this was checking again and again the same folders.

Fix this by keeping track of the visited instance root and not visit the
same instance root twice.
10 jobs for fix/prune_recursive_return in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS-Master.UnitTest-Master

01:15:47

failed SlapOS-Master.UnitTest-Master

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:44:49

passed SlapOS.Eggs.UnitTest-Master.Python3

01:17:40