Move cfg file creation to driver

Added code to load the file backend as a driver in engine __init__,
and a function in FileBackend to create cfg files.

Also added extra field in engine.cfg to specify what kind of backend
to use.

Change-Id: I6d3f24d4f676c72c94afff2c4c7f54a35cf1d4b1
Merge "Move cfg file creation to driver"