Read the settings from a yaml configuration file.

Issue: https://openedx.atlassian.net/browse/TNL-902

@e0d @jarv please review.
@jarv - it is not present in edx-platform because it is on Django 1.4, it was not required back then.

I think ['*'] should go in the dev.py settings, and leave it undefined in default configuration  - so that we will get warnings when it is not set in yml.