allow for the namevar construction

The default type would always have a section as part of its namevar, but if
you're inheriting from ini_setting you may be modelling a flat file with no
sections.

This pushes the formation of the namevar from the section_name and setting into
a method, then demonstrates overriding it so that inherited_ini_setting can just
be

   inherited_ini_file { 'setting':
     value => '12',
   }

and continue to be purgable.