Treat empty oob_program param as default

There is currently no way to reset oob_program back to its default from
the cmdline, which causes problems for cluster-merge. This patch means
that the following now works:
  gnt-cluster modify --node-parameters oob_program=

Signed-off-by: Stephen Shirley <diamond@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>