Fixed typo in health check grace periods field.

Review: https://reviews.apache.org/r/22589
Make unknown container not a Failure for Isolator::cleanup.

Review: https://reviews.apache.org/r/22049
Add a cgroup perf_event isolator.

This isolator only creates the perf_event cgroup and places the executor
into the cgroup; no performance statistics are gathered/exported.

Review: https://reviews.apache.org/r/20963
Introduce a PerfStatistics protobuf.

Review: https://reviews.apache.org/r/21442
Support for running "perf stat".

Perf can be run against either a set of pids or a set of perf_event cgroups.

Review: https://reviews.apache.org/r/21443
Add sampling support to the perf_event isolator.

Review: https://reviews.apache.org/r/21451
Initial import for health check
Fixed Protobuf to JSON serialization of optional fields with default values.

Previously, only optional fields which where explicitly set would be serialized regardless whether default values was present.
This patch adds those fields explicitly.

Review: https://reviews.apache.org/r/21246
Cleaned up JSON work
Snapshot of current health check program.
Add task health check with custom command support