fix(update_engine_stub): Set default OEM_ID to "diskless"
fix(update_engine_stub): Set default OEM_ID to "diskless"

Since writing this we've started booting via ISOs so PXE is no longer
the only boot scenario that will trigger this stub.

mjg59/update_engine,endocode/update_engine,endocode/update_engine,mjg59/update_engine,mjg59/update_engine,endocode/update_engine