Bug 1047168 - Modify test_play_youtube to play a video file locally

This test is failing if we connect to network via cell data. It works if If we're running on B2G desktop or if we have a device connected to the same network as the host via WiFi.
We should add a condition here. See https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/tests/functional/browser/test_browser_navigation.py#L19 for an example