proxy: Support slave removal

/reviewed-on !179
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

00:26:52

failed SlapOS.Eggs.UnitTest-Master.Python3

00:20:16

failed SlapOS.Eggs.UnitTest-Master.Python2

00:27:19

passed SlapOS.Eggs.UnitTest-Master.Python3

00:18:12