Add the PXE VendorPassthru interface to PXEDracDriver

Without the PXE VendorPassthru interface to expose the "pass_deploy_info"
method in the vendor_passthru endpoint of the API the DRAC it can't
continue the deployment after the ramdisk is booted.

Closes-Bug: #1379705
Change-Id: I21042cbb95a486742abfcb430471d01cd73b3a4a
(cherry picked from commit 78ec7d5336eb65ff845da7ea9f93d34b402f5a0f)