slapos-sr-testing: Fix default parameters handling

If `only-sr` is not provided, all the SR tests should run.

Fixes merge request !1011
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Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

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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

passed SlapOS.Eggs.UnitTest-Master.Python3