Decrease the number of workers

[supermamie]

In conf/lufi.conf.template the number of hypnotoad workers is set to 30, wich is, according to the documentation, a number adapted for 15 cores.

As yunoHost is made for self-hosting, I think that most people have way less cores available.
I set it to 2 (even if I have 2 cores) and the ram usage decreased from 2.5Go to 250Mo, and the load of the server dropped. (and PostgreSQL usage dropped too)

I suggest to set it down to 2 with a description in the project/admin page so users can set it up if needed.