Don't read the image to be signed from docker.io

Instead, use an already existing one in the internal registry, and hope
that we still have enough free signature slots left.

Signed-off-by: Miloslav Trmač <[email protected]>

Author: mtrmac

test/extended/images/signatures.go

@@ -11,6 +11,7 @@ import (
 	e2e "k8s.io/kubernetes/test/e2e/framework"
 
 	exutil "github.com/openshift/origin/test/extended/util"
+	"github.com/openshift/origin/test/extended/util/image"
 )
 
 var _ = g.Describe("[sig-imageregistry][Serial][Suite:openshift/registry/serial] Image signature workflow", func() {
@@ -101,7 +102,7 @@ var _ = g.Describe("[sig-imageregistry][Serial][Suite:openshift/registry/serial]
 			"--dest-creds=" + user + ":" + token,
 			// TODO: test with this turned to true as well
 			"--dest-tls-verify=false",
-			"docker://docker.io/library/memcached:latest",
+			"docker://" + image.ShellImage(),
 			"docker://" + signedImage,
 		}, " "))
 		fmt.Fprintf(g.GinkgoWriter, "output: %s\n", out)