Changed cgroups killTasks() sequence.

The sequence is as follows:
  stop -> kill -> empty -> freeze -> kill -> thaw -> empty

We also now ignore ESRCH errors from kill() in cgroups::kill().

Review: https://reviews.apache.org/r/11131
Fixed Jenkins to customize the maximum number of executors per slave.

Review: https://reviews.apache.org/r/12914
Moved Jenkins MesosCloud configuration options into advanced section.

Review: https://reviews.apache.org/r/12927