Install an sshd PAM config definition

we leverage pam_access to ensure that the ec2-user and members of the systems group and always login

we use nologin on the login nodes during maintaince to retrict user login

we use pam_slurm_adopt on the compute nodes so that only users with running jobs can login a given node.

default is to configure as a login node. Use the variable computenodepam to config as a compute node (i.e. enable pam_slurm_adopt)