Add another example to file_secontext.yml

Demonstrate what happens when there is no default context in the policy.

examples/playbooks/file_secontext.yml

@@ -9,4 +9,10 @@
       action: file path=/etc/exports seuser=unconfined_u
     - name: Set selinux context back to default value
       action: file path=/etc/exports context=default
-  
+    - name: Create empty file
+      action: command /bin/touch /tmp/foo
+    - name: Change setype of /tmp/foo
+      action: file path=/tmp/foo setype=default_t
+    - name: Try to set secontext to default, but this will fail
+            because of the lack of a default in the policy
+      action: file path=/tmp/foo context=default