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>