Fixes and add smoke test: hard timeout on isolated task.

Fixes and add smoke test: hard timeout on isolated task.

- task_runner: Only send SIGKILL once after grace period. It would previously
  call kill() in a closed loop.
- task_runner and bot: Reduce stdout logging, it made looking at logs much
  harder as the data was duplicated across stdout and the log.
- local_smoke_test: Add support to leak directories in local_smoke_test.
- local_smoke_test: Add two isolated tasks with hard timeout; one with and one
  without signal handling, to catch more corner case. This permits exhibiting a
  bug in the code base.
- Reduce number on unnecessary try/except, subprocess42.Popen is less insane.
- logging_utils: Remove confusing inheritance.

R=vadimsh@chromium.org
BUG=

Committed: https://github.com/luci/luci-py/commit/49696b59e73ee93a03111ef2461e2eebea7577c4

[M-A Ruel, 2015-10-01 13:32:51]

Patchset #1 (id:1) has been deleted

[M-A Ruel, 2015-10-01 13:34:50]

It's not fixing anything but document more the current behavior and adds hint to
make further debugging easier. It could have been 6 CLs but I didn't feel it was
worth splitting since it's <150 lines total.

[Vadim Sh., 2015-10-01 17:34:52]

lgtm

[M-A Ruel, 2015-10-01 17:48:12]

The CQ bit was checked by maruel@chromium.org

[commit-bot: I haz the power, 2015-10-01 17:48:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1373133004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1373133004/20001

[commit-bot: I haz the power, 2015-10-01 17:51:32]

Committed patchset #1 (id:20001) as
https://github.com/luci/luci-py/commit/49696b59e73ee93a03111ef2461e2eebea7577c4