Implement support for digests in imageTags

This commit adds a new field to imageTags transformer
so that digests can be used instead of image tags if needed.

Closes https://github.com/kubernetes-sigs/kustomize/issues/326

pkg/types/kustomization.go

@@ -176,4 +176,8 @@ type ImageTag struct {
 
 	// NewTag is the value to use in replacing the original tag.
 	NewTag string `json:"newTag,omitempty" yaml:"newTag,omitempty"`
+
+	// Digest is the value used to replace the original image tag.
+	// If digest is present NewTag value is ignored.
+	Digest string `json:"digest,omitempty" yaml:"digest,omitempty"`
 }