Fixed test namespace setter compilation exclusion on OSX.

Review: https://reviews.apache.org/r/27395
os::killtree() supports case when root pid has exited.

Improve os::killtree() for the case when the root pid has exited and
it has been instructed to either follow process groups or sessions. It
will now kill all processes (and their trees) with pgid or session
matching the root pid.

Review: https://reviews.apache.org/r/27295
Introduced DockerContainerizerProcess::Container::name.

Review: https://reviews.apache.org/r/26611
Introduced DockerContainerizerProcess::Container::create.

Simplifies the generic setup of a new container that currently
consists of making sure stdout/stderr exists and the directory is
properly chown'ed.

Review: https://reviews.apache.org/r/26612
Simplified launch continuations in DockerContainerizerProcess.

Combined redundant code when launching a Docker container with and
without a task.

Review: https://reviews.apache.org/r/26613
More simplifications of Docker container launching.

Review: https://reviews.apache.org/r/26614
Even more simplification of launch paths in Docker containerizer.

Review: https://reviews.apache.org/r/26615
Minor cleanup when creating Docker containers.

Review: https://reviews.apache.org/r/26616
And more refactoring in the Docker containerizer to simplify.

Review: https://reviews.apache.org/r/26617
Removed unnecessary parameter.

Review: https://reviews.apache.org/r/26618
Use the right resources when starting Docker container.

Review: https://reviews.apache.org/r/26619
Removed redundant environment for Docker containers.

Review: https://reviews.apache.org/r/26620
Replaced all tabs with spaces.

Review: https://reviews.apache.org/r/26621
Fixed non-deterministic test.

The DockerContainerizerTest.ROOT_DOCKER_PortMapping test was doing a
Docker::kill on the container which should be finishing properly which
was causing the call to Docker::kill to fail when the container had
already finished properly and been cleaned up, thus causing the test
to fail.

Review: https://reviews.apache.org/r/26630
Fixed typos in user facing documentation.

Review: https://reviews.apache.org/r/27419