Significant refactoring of run_isolated.

Significant refactoring of run_isolated.

Most of the CL is moving code around; Refactor run_isolated to be more readable.

The only functional change:
Reports all metadata about the child process via json. Change the run_isolated
process exit code to only be set when there was an internal failure. This
permits clean passing of failure/internal_failure information to task_runner.
Update task_runner accordingly.

In a follow up, this json file will be used to pass statistics so this can be
piped up to the Swarming server.

R=vadimsh@chromium.org
BUG=https://github.com/luci/luci-py/issues/197

Committed: https://github.com/luci/luci-py/commit/c41a8d02dd69f61e00c74769449cc746b38667f7

[M-A Ruel, 2015-09-15 13:50:12]

Patchset #1 (id:1) has been deleted

[Vadim Sh., 2015-09-15 17:56:33]

lgtm

[M-A Ruel, 2015-09-15 18:00:17]

The CQ bit was checked by maruel@chromium.org

[commit-bot: I haz the power, 2015-09-15 18:00:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1342673003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1342673003/20001

[commit-bot: I haz the power, 2015-09-15 18:03:17]

Committed patchset #1 (id:20001) as
https://github.com/luci/luci-py/commit/c41a8d02dd69f61e00c74769449cc746b38667f7