ssh-pregen-hostkeys: Add a recipe with pregenerated ssh host keys

Host keys are getting bigger and taking an ever increasing amount of time
to generate. Whilst we do need to test that works, we don't need to test
it in every image. Add a recipe which can be added to images with
pre-generated keys, allowing us to speed up tests on the autobuilder
where it makes sense to.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>