apt::force: Added 2 parameters for automatic configuration file handling...

...; fixes #modules-1306

when updating or installing newer packages with apt::force and you have changed previous
configuration files aptitude or apt-get will prompt what to do. You can suppress that
by pre-define the action with cfg_files parameter (new, old or unchanged and its backward
compatible if not defined). With a second optional parameter cfg_missing you can force
your provider to install missing configuration files as well.

Signed-off-by: Martin Seener <martin@seener.de>
I think rather than having invalid options silently ignored I'd like to see validation like:
```puppet
validate_re($cfg_files, '^(new|old|unchanged)$', "${cfg_files} is not supported for \$cfg_files. Valid values are 'new', 'old', and 'unchanged'.")
```

Validation should happen up towards the top of the define, probably around L11.