Increased the registry store timeout for tests.
Added a default master work directory for local runs.

Review: https://reviews.apache.org/r/23101
Added check for libnl headers in configure.ac.

Review: https://reviews.apache.org/r/23132
Enabled the support for creating nested cgroup structures. This is
important for systemd support.

Review: https://reviews.apache.org/r/22977
Do not append endln on cgroup write.

Minor update to remove the endln appending to cgroup writes. This
causes issues in later kernels, I uncovered it in rawhide.

Review: https://reviews.apache.org/r/22979
Added documentation for authorization.

Review: https://reviews.apache.org/r/23104
Updated the documentatin of SchedulerDriver::stop().

Review: https://reviews.apache.org/r/23143
Temporarisly disabled MasterTest.OrphanTasks tests due to MESOS-1543.
Fixed spelling of "Authentication" in CHANGELOG

Fixed spelling of "Authentication" in CHANGELOG for 0.19 slave authentication feature.

Review: https://reviews.apache.org/r/22257
Added 'bool' return value to Containerizer::launch.
Added composing containerizer and --containerizers flag.
Added a 'Docker' abstraction.
Added a Docker containerizer.
Bug fixes and cleanups in Docker abstraction.
Integrated DockerContainerizer.
Updated an example to use ContainerInfo to run Docker.
Add environment test for docker
Added pid() in docker/docker.cpp to get the pid of the container.
add ps option
Added rm() in docker.hpp/cpp to enable users remove containers.