Handle nested lists in cartesian().

Allow nested lists as parameters to cartesian(), and flatten them
before use.  This can make it easier to combine lists to "supersets"
(although no attempt is made to uniquify the elements of the lists).

This was originally commit a29c5ba921cd in nsc-puppet-utils.

plugins/puppet/parser/functions/cartesian.rb

@@ -19,7 +19,8 @@ module Puppet::Parser::Functions
               'iptables -i eth3 -o eth7 -j REJECT' ]
 
         A simple string as an argument is equivalent to a one element
-        long list of that string.
+        long list of that string.  Lists within lists are also allowed,
+        and will be flattened before use.
 
         If any of the arguments is an empty list, the return value will
         be an empty list.
@@ -27,9 +28,7 @@ module Puppet::Parser::Functions
     do |args|
         result = [ "" ]
         args.each do |a|
-            if not a.is_a?(Array)
-                a = [a]
-            end
+            a = [a].flatten
             newres = []
             result.each do |p1|
                 a.each do |p2|