Fixed spotty CanSupport for PV and added more unit tests for same

pkg/volume/iscsi/iscsi_test.go

@@ -37,6 +37,9 @@ func TestCanSupport(t *testing.T) {
 	if plug.Name() != "kubernetes.io/iscsi" {
 		t.Errorf("Wrong name: %s", plug.Name())
 	}
+	if plug.CanSupport(&volume.Spec{Name: "foo", VolumeSource: api.VolumeSource{}}) {
+		t.Errorf("Expected false")
+	}
 }
 
 type fakeDiskManager struct {