Reaped the container process directly in Docker executor.
Reaped the container process directly in Docker executor.

Due to a Docker issue (https://github.com/moby/moby/issues/33820),
Docker daemon can fail to catch a container exit, i.e., the container
process has already exited but the command `docker ps` shows the
container still running, this will lead to the "docker run" command
that we execute in Docker executor never returning, and it will also
cause the `docker stop` command takes no effect, i.e., it will return
without error but `docker ps` shows the container still running, so
the task will stuck in `TASK_KILLING` state.

To workaround this Docker issue, in this patch we made Docker executor
reaps the container process directly so Docker executor will be notified
once the container process exits.

Review: https://reviews.apache.org/r/65518

dforsyth/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos,andschwa/mesos,Gilbert88/mesos,andschwa/mesos,gsantovena/mesos,Gilbert88/mesos,kaysoky/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,asamerh4/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,gsantovena/mesos,Gilbert88/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,reneploetz/mesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,asamerh4/mesos,andschwa/mesos,abudnik/mesos,asamerh4/mesos,andschwa/mesos,asamerh4/mesos,kaysoky/mesos,chhsia0/mesos,asamerh4/mesos,asamerh4/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,chhsia0/mesos,jpeach/mesos,asamerh4/mesos,chhsia0/mesos,reneploetz/mesos,jpeach/mesos,andschwa/mesos,kaysoky/mesos,jpeach/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos,jpeach/mesos,jpeach/mesos