MA-81 Honor video download setting with VAL-enabled videos.

https://openedx.atlassian.net/browse/MA-81

@ormsbee Please review.
Note to reviewers: I chose to updated the existing `test_get_html_with_mocked_edx_video_id` method to test with `download_video` set to `false`.  The other `test_get_html_with_existing_edx_video_id` method tests with `download_video` set to `true`.