Revert image digest to manifest digest, Switch to use digest.Hex inst…

[akiioto]

…ead of digest.String()

Description

Changes proposed in this pull request:

• Revert image digest to manifest digest
• Use SHA value, instead of combination of algorithm and SHA value
• ...

Related issue(s)

[akiioto]

Changes between the original commit

I583797@L9KY260L6L testinfra % git diff HEAD 1b20062 -- pkg/sign/notary.go

diff --git a/pkg/sign/notary.go b/pkg/sign/notary.go
index 4285d72a4..7057c7430 100644
--- a/pkg/sign/notary.go
+++ b/pkg/sign/notary.go
@@ -140,7 +140,7 @@ func (mw *ManifestWrapper) GetConfigSize() int64 {
 
 // GetConfigDigest returns the digest of the image config.
 func (mw *ManifestWrapper) GetConfigDigest() string {
-       return mw.manifest.Config.Digest.Hex
+       return mw.manifest.Config.Digest.String()
 }
 
 // PayloadBuilderInterface defines the method for constructing the signing payload.