tools: ynl: load jsonschema on demand

The CLI script tries to validate jsonschema by default.
It's seems better to validate too many times than too few.
However, when copying the scripts to random servers having
to install jsonschema is tedious. Load jsonschema via
importlib, and let the user opt out.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>