PXE clean_up() to remove the pxe_deploy_key parameter

The pxe_deploy_key is an internal attribute used by the PXE driver to
build the pxe configuration files, it's added to the node's driver_info
in the moment that the configuration files are being generated but it's
not being removed after the node's unprovisioned. So this patch is making
sure that the pxe_deploy_key is removed from the driver's parameters at
the clean_up() method of the pxe driver.

Change-Id: Ic5f13f247d835707ab16b2cc6579c00504744fa8
Merge "PXE clean_up() to remove the pxe_deploy_key parameter"