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.
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

05:05:42

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:43:26

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

05:09:43

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:46:19

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

05:08:48

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

05:27:06

failed SlapOS.SoftwareReleases.IntegrationTest-luke/feature/kvm-drop-default-image

04:54:52