Rename CgroupsLauncher to LinuxLauncher.

This is a rename only; there are no code changes but this facilitates a
follow on review which will modify the launcher to support both Linux
cgroups and Linux namespaces.

Review: https://reviews.apache.org/r/20025
Collect Isolator::cleanup() before completing container destroy.

This ensures all isolators complete cleanup before the container
termination is set.

Review: https://reviews.apache.org/r/20720
Improve test Cluster::shutdown to destroy containers.

Extends Cluster::Slaves::shutdown to query and destroy the
containers for any slaves in the cluster, ensuring isolators can
properly clean up containers.

This is not done for slaves that are stopped so any test that completes
without slave(s) but with containers must clean up containers manually,
e.g., see the RemoveNonCheckpointingFramework test.

Review: https://reviews.apache.org/r/20430/
Added version to webui.

Review: https://reviews.apache.org/r/20863
Removed ExternalContainerizer.Launch test for disabled Python builds.

Currently ExternalContainerizerTest.Launch relies upon a Python script
which in turn relies upon the Mesos Python egg being available. When
Python is disabled (--disable-python) during the configuration phase,
the Mesos Python egg is not being built, hence the Python script
(test_containerizer.py) can not use it and the test fails.

This patch excludes ExternalContainerizerTest.Launch from the test-suite
when Python has been disabled.

Review: https://reviews.apache.org/r/20823
Modified Isolator API for prepare commands.

Isolators can return optional commands that are run in the child before
exec'ing the executor.

Review: https://reviews.apache.org/r/20026
Revised Group to drop queued events from a previous ZooKeeper instance (and ZK session).

Review: https://reviews.apache.org/r/20861