snap: Ship default configuration file for pi

Display should be rotated manualy using:

sudo snap set mir-kiosk display-config=...

I an wondering how can this be done in images
without duplicating gadget snap.

Audio is also enabled

Relate-to: https://github.com/adoptware/pinball/issues/21


Signed-off-by: Philippe Coval <rzr@users.sf.net>
Change-Id: Ib61e5084b80169c545fee3aab6b3b56b34a90092