Add a NFS server to the composition

For the moment, one only have one node which write the result onto its own filesytem. However, we want to setup a distibuted filesytem to evaluate its performances. We will take the example of NFS for this tutorial.

The approach will be the following:

1. add a new role server to the composition

2. setup the NFS server on the server

3. mount the NFS server on the compute nodes