Django: command option to operate on test database

This adds a "-T" flag to the harvest command which makes it operate on the Django test database(s). This can already be achieved using the "runserver" hooks in terrain, but I think this is cleaner. 

I'm not sure if this is still wanted, since I saw <a href="https://github.com/gabrielfalcao/lettuce/pull/298">another pull request</a> by <a href="https://github.com/skoczen">skoczen</a> that fixes <a>https://github.com/gabrielfalcao/lettuce/issues/73</a>. It seems the concern there was that the behavior was made default and included another change that switched the live server.