After function autoscaling support, it's impossible for qinling-engine
to get execution log because it doesn't know which pod it is talking
to.
So, it's neccessary for the runtime to return execution logs to
qinling engine.
The qinling client is not affected.
Change-Id: I96dfd00cc83d8b8a5e8c601ee3800b1ef1a45b1b