repman: fix problem while requesting slave with cluster bad cluster name

If cluster name is not found, use default cluster name
8 jobs for master in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:06:52

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:03:21

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:29

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:18

passed SlapOS.SlapRunner.ResilienceTest-Master

00:23:07

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:29:53

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

06:31:23

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

07:36:54