support mysql options files in reset_db

Django supports mysql option files
(see https://docs.djangoproject.com/en/dev/ref/databases/#connecting-to-the-database).

This commit adds support for pulling in connection data from a
mysql options file for `reset_db`. This code follows the priority
order as given in the documentation above:

>    1. OPTIONS.
>    2. NAME, USER, PASSWORD, HOST, PORT
>    3. MySQL option files.