In the future we want to use dry-run and subiquity internals to
do more robust validation of autoinstall user-data. Today the CI
isn't ready for this and we should rely on old behavior to not
regress CI results. This effectively moves current behavior behind
the --legacy flag.

With moving to make the validation script more user facing, we
don't need users to have the documentation link in their autoinstall
file. Add a hidden flag, --check-link, to be used in CI to validate
rendered autoinstall config has the documentation link.

./scripts/validate-autoinstall-user-data is used by the integration
tests to verify the rendered user data validates against the combined
JSON schema, but we have introduced run-time checks for more things
than can be caught by simple JSON validation (e.g. warns/errors on
unknown keys or strict top-level key checking for supporting a
top-level "autoinstall" keyword in the non-cloud-config delivery
scenario). Now the validation logic relies on the server validation
logic directly to perform pre-validation of the the supplied autoinstall
configuration.