How is self discipline a quality of leadership?

It depends, since self-discipline can mean many things in a leadership position (and it depends on the position too). Self discipline can mean the drive to get the work done in a timely manner with high effort. It can also mean that the person did not engage in any vices (alcohol, sex, drugs) and kept a clear head during an important time (mission, war, research project, etc.). Finally, self-discipline can be a quality of leadership because it can mean that you are independent but still a functioning member of a group. In other Words, you don't bring the group down, nor do you allow yourself to be brought down when the group behaves in a manner such as what I described above (lazy, having a lot of sex or alochol) and instead kept looking ahead and doing what you had to do to get the job done. Hope this helped!