Disabling Log Out button causes Power Button icon and functionality to disappear.
As reported in https://bugzilla.gnome.org/show_bug.cgi?id=794082:
Using "dconf write /org/gnome/desktop/lockdown/disable-log-out true" causes Power Button icon and functionality to disappear.
The reason seems to be that "org.gnome.SessionManager.CanShutdown" takes 'disable-log-out' option to consideration, but this brakes the whole "disable-log-out" purpose.