Image manifest JWS verification

[hickeng]

Story
As a user I want to know that the image I pull has not been tampered with

Details
Implementation of the JWS signature validation on image manfiests should be performed in the lib/imagec code.
The digest for the image layers is already computed and verified by the portlayer.WriteImage call

• verify that the layer digests are passed to this call

There should be no provision for accepting an image that fails signature validation if a signature is present.

Acceptance

• layer with checksum that does not match the layer digest - should be rejected
• image manifest that fails validation via JWS signature - should be rejected
• layers and images with correct digests should be accepted

bug1727662

[zjs]

Summary of discussion: Implement Notary instead of this (if it's an either-or decision). The VCH should have a Notary client built in to verify integrity. See #4450.