`slurmd_sysconfig` may not work for Debian users when `slurm.install` is false

Native Debian (and also Ubuntu) use `/etc/default/slurmd/` as system config path, instead of RHEL's `/etc/sysconfig/slurmd`.
Therefore, if user use native Slurm, the following code may useless.

https://github.com/Azure/cyclecloud-slurm/blob/c5de6d68fe0c5319fdd44c2f367509c85bf3c135/specs/default/chef/site-cookbooks/slurm/recipes/execute.rb#L112-L121

An fix can be, when `node['slurm']['install'] == true` and `/etc/default/slurmd/` directory exists, write file into this directory instead.