silverblue,ks: quote keyboard's argument

keyboard command (from kickstart) receives a single parameter as
argument, thus whatever we receive as l10n-keyboard must be quoted as
applications may just set things as 'in (eng)'.

The same approach is already taken on Fedora kickstarts.

https://bugzilla.redhat.com/show_bug.cgi?id=1666320



Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>
Reviewed-by: Cole Robinson <crobinso@redhat.com>