GlideinWMS

Currently, the support for user job wrappers in GlideinWMS allows multiple wrappers but they are "cat" one after the other without control and coordination.
The first one exec-ing the job (like wrappers normally do) is then interrupting the chain.

The idea is to allow VO to have custom wrappers that are loaded as modules by the GlideinWMS job wrapper
Ideally, these modules could run outside and/or inside Singularity (the container)
This would be especially useful to implement pre-job tests when using Singularity.