Add user lookup and session dependencies to systemd service file.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1088619
    
    Before permitting user sessions sssd should be running. This also correctly
    orders shutdown of sssd after the user sessions.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit ea0a71921ea396f5cc0e9e20d9a2aafc681b3eb9)