lxc/create-testbed: avoid cleaning up valid containers
Sometimes, the creation of a new testbed fails for various reasons (some essential package) breaks debootstrap, or something fails to download. Those failures are already handled by autopkgtest-build-lxc: the old container is only replaced by the new one in case of success. In that case, it's OK for debci to just keep using the older container since it will be upgraded before running any tests anyway. The only case where we indeed want to remove containers is when the customizations needed by debci are not successfully applied.
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