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

This is a work-in-progress, but I've verified through 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

As ec2 has some really old unit tests, there's still some grunt work needed to complete this change.