settings per deploy target

This adds the possibility to define settings per target on the scrapy.cfg.

If you run several scrapyD instances, you can define different settings module for each of them in your scrapy project. This is useful test things in a development/testing environment before sending them to a production environment.

My scrapy.cfg looks like this:
```ini
[deploy:live]
url = http://live.mysite.com:6800/
project = myproj
settings = myproj.settings_live

[deploy:tests]
url = http://tests.mysite.com:6800/
project = myproj
settings = myproj.settings_tests
```

Cherry pick the change set,  only changes in deploy.py are relevant