Allow using a default tools tree

Allow using a default tools tree

Instead of requiring users to always set up a tools tree preset, let's
allow specifying "default" as the tools tree to have mkosi build a default
tools tree itself. This default tools tree includes all the software that
might be necessary to build an image (excluding software that might be
required by various user scripts).

For distributions that do not have a rolling release variant, we use the
closest matching distribution. e.g. for CentOS, we use Fedora and for
Ubuntu we use Debian.