Allow configuring more plugins in plugstack.d/ or plugstack.conf.j2

[martbhell]

https://github.com/CSC-IT-Center-for-Science/ansible-role-slurm/blob/77-slurm-non-backwards-compatible/tasks/common.yml#L63

Only templates in plugstack.d/x11.conf

Perhaps one of these? Any preference? I think A) looks fairly easy
A) via variables template in a more rows in plugstack.conf.j2 itself
B) template in a more general plugstack.d/plugstack_ansible.conf where one with variables can define extra settings.

One example usecase: https://github.com/singularityware/singularity/tree/master/src/slurm