Update Image Signature policy

• The signature parameter key/values changed upstream. Updated the default values/test-values to align with the latest format.
• Added IronBank signature checking to test-values (we want to know if this doesn't work in our testing)
• Turned on the signature policy in test-values so it would be checked
• Turned off digest verification and mutation for IronBank (we don't want the tag converted to a SHA since IronBank rebuilds images nightly)
• Added test vector to cover non-signed, non-verified image since IronBank images are signed/verified now.

Related to #22 (closed)