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:

    OPTIONS.
    NAME, USER, PASSWORD, HOST, PORT
    MySQL option files.