To fix bugs in CameraManager.py

[fracarnicelli]

In "CameraManager.py" there's the need to fix the management of Asset path in order to use the right photo for testing.

[GassiGiuseppe]

It is referring to this variable here: https://github.com/gdgbari/photobooth/blob/master/CameraManager.py#L71 .
it belongs to the method get_fake_shoot() which is called during testing when a camera is not connected to the pipeline, here a props of the photo is forced as an output, i think that simply it can be locally changed to the wanted path without adding further logic to the application