Set PAM to only allow ssh from users with running jobs

[sjpb]

As per S3.8.4.4 of the openhpc v2 install guide, should configure PAM so that only users with a running job can login to a compute node.

[sjpb]

See some helpful comments here about what the different pam modules do

[sjpb]

Things todo:

• Create a non-centos/non-wheel test user
• In /etc/pam.d/sshd" add

  account    sufficient   /lib/security/pam_access.so
  account    required     /lib/security/pam_slurm.so

• In /etc/security/access.conf: +:wheel:ALL -:ALL:ALL

[sjpb]

Done by https://github.com/stackhpc/openhpc-demo/commit/4105e3ca78a71c0b044b473ca8d6f3af0929182a