grid: Fix OS detection

See merge request !192
15 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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