load per-site configuration file

Right now, we can deal with different site inside the same config file but as the configuration grows it can become messy.
It'd be cleaner to be able to load per-site configuration file.
We just a need to add some logic :
The config file name is $PRODUCTNAME.yaml, but if there is a config file named $PRODUCTNAME$SITE.yaml_, (for example: drp_L.yaml)
then you load this one.