Update Nova Ironic Driver destroy() method
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

openstack/ironic,bacaldwell/ironic,Tan0/ironic,supermari0/ironic,naototty/vagrant-lxc-ironic,devananda/ironic,NaohiroTamura/ironic,ionutbalutoiu/ironic,SauloAislan/ironic,dims/ironic,pshchelo/ironic,pshchelo/ironic,ionutbalutoiu/ironic,ramineni/myironic,rdo-management/ironic,NaohiroTamura/ironic,froyobin/ironic,Tehsmash/ironic,naterh/ironic,naototty/vagrant-lxc-ironic,hpproliant/ironic,bacaldwell/ironic,openstack/ironic,redhat-openstack/ironic,dims/ironic,debayanray/ironic_backup,SauloAislan/ironic,rackerlabs/ironic,faizan-barmawer/openstack_ironic