Implement /nodes/UUID/vendor_passthru in the API

The RPC and Conductor layers are already done;
this patch adds some exception handling and implements the
RPC bindings inside the API service so that /vendor_passthru
will be possible.

Change-Id: Iba3feae0445a0eb202efc5b9a346fb9931dc4b00
Merge "Implement /nodes/UUID/vendor_passthru in the API"