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
Merge "Correctly munge glance_image is_public property"