Version 1 of install-sources.yaml is a top-level list of the sources to
be offered.
Version 2 extends this by placing the list under a top-level key
`sources`, adding a `version` field, and adding a `kernel` field which
supplants the current kernel-meta-package file. `kernel.default` is
read to know which kernel to use - unless we need to fallback to the
bridge kernel.
The ubuntu-core-installer image is an installer that installs ubuntu
core. The environment the installer runs in is similar to the server
installer but it has a source catalog entry that points to the model
created in ubuntu-core-installer/hooks/05-prepare-image.binary, which
subiquity knows how to install.
