JCLOUDS-480 start supporting version 4 signatures for aws-ec2.

I've verified through mock and manual testing that it works in frankfurt. The code is a variant of something I've created in the past https://github.com/Netflix/feign/blob/master/sax/src/test/java/feign/sax/examples/AWSSignatureVersion4.java

Old aws-ec2 tests that depend on signature logic have either been rewritten as MockWebServer tests, or ported.