Currently, setting the $custom_preferences variable to false fails
because of the /etc/apt/preferences concatenated_file being re-defined.
Fix this by setting dependencies on the resource only when we expect to
create the /etc/apt/preferences file. Also, since there is no "ensure"
parameter to concatenated_file, use a simple file resource to ensure
that it is removed.
Signed-off-by: Gabriel Filion <lelutin@gmail.com>