Added driver and unittests for CloudFront

I just noticed this https://github.com/rackerlabs/cdn/blob/master/tests/unit/provider/fastly/test_services.py - which aligns with how you have written this test. @amitgandhinz, @tonytan4ever, @obulpathi --> Thoughts on breaking up these huge tests (tht cover assertions, exceptions & happy paths) into multiple tests?