Make PhutilAWSEC2Future signs requests correctly

Summary: I partially automated a bit of provisioning stuff; the signature algorithm for EC2 is slightly different from the algorithm for S3.

Test Plan: Used bin/provision ... in rCORE to create and attach device volumes in the Phacility cluster.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16963