software/kvm: Drop default image and replace it with boot-image-url-select

boot-image-url-select is used instead of default image being downloaded by the
software release.

If nothing is selected, the default boot-image-url-select is used, but not if
other way to obatin boot image is enabled.
8 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:13:24

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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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.Python3Next

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

11:17:37

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:56:45

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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