svcbackend/standalone: use shorter names for supervisor sockets

On test nodes, these socket path sometimes exceed the linux limit.
15 jobs for fix/testcase-fixes in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:34:05

passed SlapOS.Eggs.UnitTest-Jerome.Python3

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failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

13:14:29

failed SlapOS.Eggs.UnitTest-Jerome.Python2

00:38:41

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:22:06

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:22:36

failed SlapOS.Eggs.UnitTest-Jerome.Python2

00:32:16

failed SlapOS.Eggs.UnitTest-Jerome.Python2

failed SlapOS.Eggs.UnitTest-Jerome.Python2

00:25:30

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:41:22

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:32:54

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:10:35

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:11:33

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:13:26

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:11:34