Simplify _run_command
    
    Either TTY or non-TTY, _run_command does same thing that run a single
    command or pipe command. This change simplifies _run_command to run
    commands for both TTY or non-TTY. As a result, no output to stdout and
    stderr is captured, user is able to redirect output to a destination by
    himself.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>