add an array_join function

lib/puppet/parser/functions/array_join.rb

@@ -0,0 +1,8 @@
+Puppet::Parser::Functions::newfunction(:array_join,
+    :type => :rvalue,
+    :doc  => "Join a list of strings with an optional delimiter.") do |args|
+  unless 1..2.include? args.length
+    raise Puppet::ParseError, 'array_join() takes 1 or 2 arguments'
+  end
+  args.first.join args.last
+end