Enable an optional global cache directory

This commit introduces a new parameter 'cache_dir' for the wget class
which is undef by default. This parameter value is used as the default
value for the wget::fetch class's 'cache_dir' parameter. This allows one
to enable download caching for all wget::fetch resources without having
to specify the cache directory in all of them.

By defining the $wget::cache_dir value through hiera one can make all
wget::fetch resources use the cache, even the ones declared on
third-party modules.