unattended-installer: Pass a better keyboard id to osinfo

We used to pass the same LANG identifier for both language and keyboard.

These changes lookup on the user's default keyboard layout and uses it for the express installs. We also make sure that the current keyboard set in the user's system's is supported by osinfo by checking against the database.

This should fix #240 (closed) and https://bugzilla.redhat.com/1666320