slapos_accounting: Dont rely on grouping_date for outstanding amount

See merge request !435
7 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS-Master.UnitTest-Master

01:29:23

passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:16