remove remote file HEAD to check if-modified-since

Issue DE126

cloudfiles does not properly honour a HEAD and an if-modified-since header (see https://github.com/rcbops/chef-cookbooks/issues/391#issuecomment-17604434)

cloudfiles does honour a GET with the if-modified-since header, but chefs http_resource seems to ignore the result anyway.

So, we'll just do a local hash comparison, and get the file each time for now.  At least it's consistent, and tbh we ended up getting the file 9 times out of 10 anyway.