apt::force: rspec: fixed the failing tests and added validate_re for cfg_files and validate_bool for cfg_missing. Also removed default values for both case statements and only allow pre-defined values or true/false. Furthermore enhanced the README refs #module-1306

Was able to fix the failing rspec tests for the patch.
Thanks to Morgan Haskel.

Signed-off-by: Martin Seener <martin@seener.de>