Replace manage.py with clearinghouse's.

The SeattleTestbed/clearinghouse and SeattleTestbed/custominstallerbuilder
Django projects are laid out somewhat differently (for reasons lost
in history). I'll try to make them more similar as I go through the
changes required for supporting Django 1.6.

In this case, this comes at the expense of forcing the user to set
up environment variables (`DJANGO_SETTINGS_MODULE`, `PYTHONPATH`)
rather than trying to configure them automatically (or so it seems
--- I'll find out when testing.) Now is not the time for me to do
porting *and* debug clever logic at the same time, so I'm going
for the simpler, non-autoconfiguring method.