Add an option to load a virtualenv site dir

I've been trying to use root installs of circus and chaussette+backend to serve my wsgi apps. So far I've only been able to have a root install of circus with chaussette+backend still inside the virtualenv with this config:

```ini
[watcher:app]
cmd = /home/user/.virtualenvs/app/bin/chaussette app.app --backend gevent --fd $(circus.sockets.app)
use_sockets = True
numprocesses = 3
copy_env = True
copy_path = True

[env:app]
PYTHONPATH = /home/marksteve/.virtualenvs/app

[socket:app]
host = 127.0.0.1
port = 8080
```

Since I mostly use similar configurations for my wsgi apps, I'd like to be able to deploy by just adding a circus config for the app pointing to its virtualenv instead of having to install chaussette+backend to the app's virtualenv everytime.

Am I missing something? Can this be done with just the config?
If not, is it a valid feature request? I'm willing to try to implement it if I'm not alone with wanting this :)