Serialize Containerizer::launch,wait invocations.

Review: https://reviews.apache.org/r/23463
Added 'bool' return value to Containerizer::launch.

Review: https://reviews.apache.org/r/23461
Added composing containerizer and --containerizers flag.

Review: https://reviews.apache.org/r/23462
Added a helper for determining the cgroup of particular pid.

These helpers perform the non-trivial steps required to determine the
cgroup of a particular pid for a particular hierarchy where a
particular subsystem is attached (in this case, we've provded
implementations for the 'cpu' subsystem hierarchy and 'memory'
subsystem hierarchy, which may be the same depending on how cgroups
are mounted).

Review: https://reviews.apache.org/r/23632
Fix stout/os to properly check errno from getpwnam_r.
Support some Linux systems (e.g., RHEL7) which return non-zero and set
errno to indicate a user or group was not found.

Review: https://reviews.apache.org/r/24249
Removed unused code from future.hpp.

Review: https://reviews.apache.org/r/23874
Made WeakFuture<T>::get a const operation.

Review: https://reviews.apache.org/r/23875
Master detector cleanups.

Review: https://reviews.apache.org/r/23866
Handle discards on the master detection futures.

Review: https://reviews.apache.org/r/23867
Added a ping timeout in the slave to trigger a re-detection.

Review: https://reviews.apache.org/r/23868
Fixed header paths for proto files during installation.

Review: https://reviews.apache.org/r/23704
Added a 'Docker' abstraction.
Added a Docker containerizer.
Bug fixes and cleanups in Docker abstraction.
Integrated DockerContainerizer.
Add environment test for Docker.
Added pid() in docker/docker.cpp to get the pid of the container.
Added Docker::ps() "all" option.
Added rm() in docker.hpp/cpp to enable users remove containers.
Modify validate Docker to call 'info' instead.