expose: Truncate service names

In case the generated service inherits the exposed object's name (the user didn't specify
a name via --name), truncate it up to the maximum length for a valid service name

hack/testdata/pod-with-large-name.yaml

@@ -0,0 +1,11 @@
+# Used for testing name truncation in kubectl expose
+apiVersion: v1
+kind: Pod
+metadata:
+  name: kubernetes-serve-hostname
+  labels:
+    name: kubernetes-serve-hostname
+spec:
+  containers:
+  - name: kubernetes-serve-hostname
+    image: gcr.io/google_containers/serve_hostname