Prevent unicode arguments being sent to twisted
Prevent unicode arguments being sent to twisted

Arguments to twisted.spawnProcess must not be unicode
and must not contain nulls. Add a checker which logs
conversions but always allows something to be passed.
Constrain ealier change to childDataReceived to only
operate when the log file would otherwise be empty.

Change-Id: I8a4ab885b786a91fbe1a1666dece31b2ba432d53

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server