Correctly munge glance_image is_public property

glance expects True/False values whereas this type uses Yes/No.

Without this patch, puppet would return the following message with an
existing glance image with is_public attribute already set to 'True'.

puppet-agent[18190]: (/Stage[main]/Site::Controller/Glance_image[raring-server-cloudimg-amd64]/is_public) is_public changed 'True' to 'Yes'

Change-Id: I532e08e6815a8b03f5eb6a9495d886b9855762c8

lib/puppet/type/glance_image.rb

@@ -46,7 +46,8 @@ Puppet::Type.newtype(:glance_image) do
     newvalues(/(y|Y)es/, /(n|N)o/)
     defaultto('Yes')
     munge do |v|
-      v.to_s.capitalize
+      'True'  if v =~ /^(y|Y)es$/
+      'False' if v =~ /^(n|N)o$/
     end
   end