Vendor after merging mtrmac/image:openpgp, + other updates

Primarily vendor after merging mtrmac/image:openpgp.

Then update for the SigningMechanism API change.

Also skip signing tests if the GPG mechanism does not support signing.

Also abort some of the tests early instead of trying to use invalid (or
nil) values.

The current master of image-tools does not build with Go 1.6, so keep
using an older release.

Also requires adding a few more dependencies of our updated
dependencies.

vendor/github.com/containers/image/copy/copy.go

@@ -235,6 +235,11 @@ func Image(policyContext *signature.PolicyContext, destRef, srcRef types.ImageRe
 		if err != nil {
 			return errors.Wrap(err, "Error initializing GPG")
 		}
+		defer mech.Close()
+		if err := mech.SupportsSigning(); err != nil {
+			return errors.Wrap(err, "Signing not supported")
+		}
+
 		dockerReference := dest.Reference().DockerReference()
 		if dockerReference == nil {
 			return errors.Errorf("Cannot determine canonical Docker reference for destination %s", transports.ImageName(dest.Reference()))