Update images.js

Hi, there is a bug in Openstack where you cannot set the metadata of os_type as it is overwritten to None when first creating the image. This causes a problem with the timezone not being set correctly on windows machines. So I wrote a function, updateImageMeta, that allows a client to update the image metadata , after calling createImage function, and wanted to share it so maybe it will be in future versions of pkgcloud and help other people who might need this functionality as well.