Add ressources usages param to global conf + rework modules conf generation

Create enum ResourceUsage with 4 variants : Minimal, Medium, Large, Infinite.

Add 4 fields to global conf :

cpu_usage : ResourceUsage network_usage : ResourceUsage memory_usage : ResourceUsage disk_space_usage : ResourceUsage

The configuration of the modules has now been generated from 2 parameters :

1. global_conf
2. module_user_conf (json::Value)

module_user_conf will only contain fields that have been manually redefined by the user - so empty by default