Add import-boss directives

The pkg/api/testing can be removed whem kubeadm splits into
its own repository.
The pkg/kubemark probably will remain as is for the foreseeable
future.

pkg/api/testing/.import-restrictions

@@ -0,0 +1,12 @@
+{
+	"Rules": [
+	  {
+		"SelectorRegexp": "k8s[.]io/kubernetes/cmd",
+		"AllowedPrefixes": [
+		    "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm"
+		],
+		"ForbiddenPrefixes": [
+		]
+	  }
+	]
+}