Different venv for gx_it_proxy

Hi!
In the admin training we set `gie_proxy_virtualenv` to a different venv than galaxy's venv
When you configure gravity, (I think) there's no way to tell it to use this specific venv only for the gx_it_proxy service
Not sure what would be the best fix for that:
- just set `gie_proxy_virtualenv` to galaxy's venv?
- set the PATH variable on the gx_it_proxy service in gravity's config?
- patch gravity to allow setting a different virtualenv at service level?

(I think it has worked for us at .fr because we accidentally did solution 1 at some point, but reinstalling galaxy's venv from scratch made this problem appear)