slapgrid: Fix promises not being logged to logfile

See merge request !356
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:27:37

passed SlapOS.Eggs.UnitTest-Master.Python2

00:11:48

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:00