This adds a basic airship-core type and test-site definition, which initially
will point back to airshipctl for everything but site-specific configuration.
Gating is added under a dependent changeset:
This removes the yaml-lint job / make target, since that chokes on CRs whose
definitions are not present. YAML validation will be re-enabled by borrowing
the YAML validation from the airshipctl project, which leverages kustomize
and a live kubernetes API server.