Allow multiple `modules_system` per system

[ekouts]

Since we support spack as a module system it might be useful to be able to use more than one per system. Maybe we could extend the module object to have an optional attribute that chooses which module system to use and by default we could use the first one in the configuration.

[ekouts]

I am not sure if we should extend the configuration schema to accept both str and list for the modules_system attribute so that the old configutation files don't "break".