fff-wireguard: rename keys to remote_public_key/local_private_key

This renames two options for wireguard interfaces in
/etc/config/gateway:
public_key -> remote_public_key
private_key -> local_private_key

Signed-off-by: Alexander Gutzeit <alexander.gutzeit@web.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>
[adjusted commit message/title, added version bump, remove
rename for network config]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>