Added Network Isolator flags to configuration.md.

Review: https://reviews.apache.org/r/29219
Added Hooks to Master.

Review: https://reviews.apache.org/r/28655
Added environment decorator for Slave.

Review: https://reviews.apache.org/r/28656
Added remove-executor hook for slave.

Review: https://reviews.apache.org/r/28875
Added example hook module.