software/helloworld: use new slap-connection name

slap_connection is a deprecated alias, we use kebab case in buildout
profiles.

/reviewed-on !688
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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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