scripts: domains: Support initialization only from YAML

By requiring a full-on YAML fragment, the Domains class can rely on
pykwalify to do the heavy lifting with validating types and structure.
Remove support for loading domains from a dictionary, so that we don't
ever have to replicate the same checks on the input, and we can instead
have a single entry point.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>