slap/standalone: Increase slapos error log size

The output of slapos commands has been observed to be truncated earlier
than desired in some cases due to long promise messages in output of
slapos node instance and long paths included in the logs.

See merge request !551
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

02:39:07

failed SlapOS.Eggs.UnitTest-Master.Python2

00:17:59

failed SlapOS.Eggs.UnitTest-Master.Python3

00:18:46