kubectl resource builder: don't check extension for single files

`kubectl create -f filename` doesn't need to check the extension
of filename. This fixes that behavior.

examples/pod

@@ -0,0 +1,13 @@
+# Copy of pod.yaml without file extension for test
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx
+  labels:
+    name: nginx
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    ports:
+    - containerPort: 80