Ec2Signer: Initial support for v4 signature verification

Adds initial support for verifying AWS v4 signatures, tested with
the latest boto trunk (which now uses v4 signatures by default)

Change-Id: Id163363e259cf08aa251a7a00ff4293b742cbef6
blueprint: ec2signer-v4signatures
(cherry picked from commit 5c37d85944d9eed73ec6dd6254842108386bcc4f)