Update Nova Ironic Driver destroy() method

* Add DocString

* Add a missing parameter 'destroy_disks', although not used by Ironic
  we should keep the function signature consistent with the base class.

* When calling destroy() passing an non-existent instance log a warning
  message instead of debug. As suggested on the base class:

  "If the instance is not found (for example if networking failed), this
  function should still succeed. It's probably a good idea to log a
  warning in that case."

Change-Id: I0e7d9a9e5522606bafd5ef6d9dbe525ad39026d2
Merge "Update Nova Ironic Driver destroy() method"