adding comment

playbooks/edx-east/remove-ubuntu-key.yml

@@ -10,6 +10,8 @@
       when: public_key is not defined
     - command: mktemp
       register: mktemp
+    # This command will fail if this returns zero lines which will prevent
+    # the last key from being removed
     - shell: >
         grep -Fv '{{ public_key }}' {{ keyfile }} > {{ mktemp.stdout }}
     - shell: >