Fixed flaky FaultToleranceTest.ReconcileIncompleteTasks test.

Review: https://reviews.apache.org/r/12064
Fixed flaky MasterDetectorExpireSlaveZKSessionNewMaster test.

Review: https://reviews.apache.org/r/12065
Fixed slave to create work directory during initialization.

Review: https://reviews.apache.org/r/12010
Added a log message in the master for tasks lost during slave removal.

Review: https://reviews.apache.org/r/12044
Improved the configure output when deprecated headers are present
on OS X.

Review: https://reviews.apache.org/r/12045
Added Jenkins scheduler.

Review: https://reviews.apache.org/r/12033
Added Jenkins plugin to the build tool chain.

Review: https://reviews.apache.org/r/12034
Fixed links in webui for command executor tasks.

Some frameworks don't report an executor ID, but have a task ID. As a
workaround, we can use the task ID when the executor ID is empty.

Review: https://reviews.apache.org/r/11814