simplify module usage by qualifying exec ourselves
The ufw class as well as all the defined types now declare a sensible
defaults for Exec:

    path     => '/usr/sbin:/bin:/usr/bin',
    provider => 'posix',

Now users can simply /use/ the class. The tests/ are adapted to
reflect this, as is the documentation.
Merge pull request #13 from igalic/qualify-exec

simplify module usage by qualifying exec ourselves