Check the etag/checksum before downloading

When downloading we can check the etag that is the same as the checksum
before using bandwidth to download files.

The tests currently fail for 2 reasons. One is the outstanding Mocks missing for Rackspace Cloud Files in fog, and two, the most recent xml cookbook is broken due to nokogiri trying to compile with system libraries no matter what the attribute is set to.

I am working on the upstream XML cookbook fix.