slap/standalone: Use IPv6 range when available

See merge request !538
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

08:43:09

passed SlapOS.Eggs.UnitTest-Master.Python2

03:10:20

failed SlapOS.Eggs.UnitTest-Master.Python3

03:25:52