Add a unit test for validateManifest which ensures extra data can't be
injected by adding data to the JSON object outside the payload area.
This also removes validation of legacy signatures at pull time. This
starts the path of deprecating legacy signatures, whose presence in the
very JSON document they attempt to sign is problematic. These
signatures were only checked for official images, and since they only
caused a weakly-worded message to be printed, removing the verification
should not cause impact.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
