generate-secrets.py now requires --development for development environment
setup or --production for production environment setup (and one of these
options is mandatory).
This solves the problem that it was somewhat easy to accidentally run
generate-secrets.py without the `-d` option while doing manual development
environment setup.
Fixes: #1911.
The manage.py change effectively switches the Zulip production server
to use the virtualenv, since all of our supervisord commands for the
various Python services go through manage.py.
Additionally, this migrates the production scripts and Nagios plugins
to use the virtualenv as well.