Fixed incorrect invocation of the mesos-executor within test_containerizer.py.

The incorrect invocation of the mesos-containerizer would possibly
trigger problems. This should get rid of failures where the slave runs
into a timeout when waiting for the executor registration on
ExternalContainerizerTest.Launch.

Review: https://reviews.apache.org/r/20819
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
Add tests/isolator.hpp to the distribution

Review: https://reviews.apache.org/r/20872
Fixed infinite recursion in test containerizer.

Review: https://reviews.apache.org/r/20870
Fixed distclean break during maven clean up.

The pom file gets cleaned up before clean-java runs, which calls maven
and fails. Solution is to just remove the target dir, which is what
maven does as well.

Review: https://reviews.apache.org/r/20874
Fixed some indentation.
Better error message for protobuf::write.

Review: https://reviews.apache.org/r/20072
Added 'mesos-usage' for use by external containerizers.

Review: https://reviews.apache.org/r/20070
Added test for slave stopping before containerizer launches.

Review: https://reviews.apache.org/r/20179
Allowed passing headers to http::get/post.

Review: https://reviews.apache.org/r/20275
Updated uses of http::post.

Review: https://reviews.apache.org/r/20278
Used memory::shared_ptr instead of boost::shared_ptr.

Review: https://reviews.apache.org/r/20305
Added 'cref' and 'ref' to lambda:: namespace.

Review: https://reviews.apache.org/r/20306
Added process::async overloads for void functions.

Also used preprocessor macros to enumerate overloads.

Review: https://reviews.apache.org/r/20307