Support content trust images and pull with authentication

Contact the notary server if ```--verify``` is specified (or `verify:
true` is enabled on the repo config) and verify if the image is signed,
use the returned value to pull the verified image.

pkg/installer/client/interface.go

@@ -18,4 +18,5 @@ package client
 type RepoData struct {
 	Urls           []string
 	Authentication map[string]string
+	Verify         bool
 }