Use imageutils instead of hardcoded image paths

A number of tests were using hardcoded image paths instead of
going through the imageutils package. The reason for centralizing
the logic there is to keep an eye on what images we use and where
they come from.

test/e2e/storage/utils/utils.go

@@ -442,7 +442,7 @@ func StartExternalProvisioner(c clientset.Interface, ns string, externalPluginNa
 			Containers: []v1.Container{
 				{
 					Name:  "nfs-provisioner",
-					Image: "quay.io/kubernetes_incubator/nfs-provisioner:v2.2.2",
+					Image: imageutils.GetE2EImage(imageutils.NFSProvisioner),
 					SecurityContext: &v1.SecurityContext{
 						Capabilities: &v1.Capabilities{
 							Add: []v1.Capability{"DAC_READ_SEARCH"},