Give bind::key the ability to generate keys

Also, allow them to be placed elsewhere in the filesystem and make it possible
to exclude a key from the named configuration.

lib/puppet/parser/functions/hmac_secret.rb

@@ -0,0 +1,9 @@
+# ex: syntax=ruby si sw=2 ts=2 et
+require 'securerandom'
+
+module Puppet::Parser::Functions
+  newfunction(:hmac_secret, :type => :rvalue) do |args|
+    bits = args[0].to_i
+    SecureRandom.base64(bits / 8)
+  end
+end