SSSD: Add a new option diag_cmd
    
    This option is an optional one that is run when a sbus ping times out
    and before a SIGKILL signal is sent.
    
    It is undocumented by default.
    
    diag_cmd (string):
    A command that should be run for diagnostic purpose when an sbus timeout
    fails. The option value may contain %p which would be expanded for the
    process ID of the process that timed out
    
    Example:
            pstack %p
    This setting would print the stackstrace of the service whose ping timed out.
    
    Default: not set.
    
    Reviewed-by: Petr Cech <pcech@redhat.com>