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.

XXX:
  * update tests for boot-image-url-select
  * cover non-default cases (boot-image-url-list, nbd-host, nbd2-host and virtual-hardrive...)