PXEAndSSH driver lacked vendor_passthru

Add vendor_passthru interface to PXEAndSSHDriver
to allow PXE-based deploys to continue.

Change-Id: Ief087f75b0b6975ed2b461f46cddcc9067316efe
Closes-bug: 1268066
Merge "PXEAndSSH driver lacked vendor_passthru"