Cleanup v2 push logic

Manifest is now generated during a v2 push, not relying on previously generated hashes. When pushing a layer, the hash is directly calculated from the tar contents which will be pushed. Computing the hash on push ensures that the hash contents always match what is seen by the registry. This also mitigates issues with tarsum differences and permits using pure SHA digests.
Additionally the new manifest function is moved to the unit tests since it is no longer called outside the tests.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

integration-cli/docker_cli_push_test.go

@@ -45,7 +45,7 @@ func TestPushUntagged(t *testing.T) {
 
 	repoName := fmt.Sprintf("%v/dockercli/busybox", privateRegistryURL)
 
-	expected := "No tags to push"
+	expected := "Repository does not exist"
 	pushCmd := exec.Command(dockerBinary, "push", repoName)
 	if out, _, err := runCommandWithOutput(pushCmd); err == nil {
 		t.Fatalf("pushing the image to the private registry should have failed: outuput %q", out)