Don't include do_task() results in the summary (fix #851)

[kablamo]

Fix for #851

How to test

• prove t/summary.t

[krimdomu]

Currently this crashes on my windows system (win 10, perl 5.22.0) (perl interpreter just dies)
The idea to define a parent task is nice and maybe we can extend this idea to print a tree like summary.

Also i'm not sure if we want to hide do_task calls in the summary. The difference between run_task and do_task is that run_task is used to run one task on one server to get some value in return. So the Rexfile should handle failures with this internally.

do_task on the other hand just run the task on all the defined servers so i think it is good to display those results.

[krimdomu]

I'm going to add this to 1.5.0 milestone, then we can think about it and how we want to display the results.