Move policy expansion into a single location.

  This is a huge change, but now the handling of the following
 line(s) now comes from Slaughter::Private:

        FetchPolicy( "foo.policy" );

   We handle the variable expansion in a single location
  and use the %CONFIG hash from the main slaughter script
 to turn "$fqdn.policy;" into "precious.my.flat.policy", for
 example.

  Added test-case to ensure it works as expected.
  perltidy