QA: Remove 'oob_program=default' on gnt-cluster modify

On cluster level there's no 'default' because it's the highest cascading
level. Due to this 'default' is a valid value and doesn't mean to remove
the value from the dict like it does on the other levels. To overcome
this we just empty the string.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

qa/qa_node.py

@@ -346,7 +346,7 @@ def TestOutOfBand():
       AssertCommand(["rm", "-f", oob_path2, verify_path2])
   finally:
     AssertCommand(["gnt-cluster", "modify", "--node-parameters",
-                   "oob_program=default"])
+                   "oob_program="])
     AssertCommand(["rm", "-f", oob_path, verify_path, data_path,
                    exit_code_path])