slapgrid: binary cache only option

/reviewed-on !173
5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

00:20:35

failed SlapOS.Eggs.UnitTest-Master.Python3

00:10:16

failed SlapOS.Eggs.UnitTest-Master.Python2

00:20:15

failed SlapOS.Eggs.UnitTest-Master.Python2

00:21:37

failed SlapOS.Eggs.UnitTest-Master.Python3

00:10:08