PAM config prevents launching of GDM on Fedora 27

[avanier]

As title suggests, Gnome Display Manager is unhappy and falls into a crash loop with the pam config provided in rhel_system_auth.erb on Fedora 27.

Essentially, it seems to be missing the following line :

-session     optional       pam_systemd.so

in the last block of rhel_system_auth.erb.

Will submit PR, but I have no idea of the security implications. The man page suggests if essentially creates the context for certain systemd units to create a context for the user to login to graphical sessions, which seems innocuous enough at first glance.

I'd conditional this with platform detection and node['os-hardening']['desktop']['enable'].