OpenStack: Set security group for Packer image build instance

[jcwomack]

Use security group from startnode configuration.

This should avoid the Packer image build instance from using the project's default security group (fixes #147).

Also:

• Explicitly install authselect package when running sssd role's tasks
  • In some Rocky 8 images, it seems that authselect is not present and needs to be installed prior to running the "Enable SSSD auth in PAM" task

[jcwomack]

I have not had a chance to test building a new compute node instance with the updated Ansible-deployed configuration due to some issues with the Digital Labs prototype OpenStack control plane. I'll try an image build using the update Packer configuration once this is resolved.

Update: I have now successfully built compute node images using the updated Ansible-deployed configuration.