Added os::execvpe to stout.

Review: https://reviews.apache.org/r/24679
Updated subprocess to use os::execvpe.

Review: https://reviews.apache.org/r/24681
Updated mesos to use os::execvpe.

Review: https://reviews.apache.org/r/24682
Fixed the flaky SlaveRecoveryTest.ShutdownSlave.

Review: https://reviews.apache.org/r/24678
MESOS-1174: Update system check to unbundle protobuf

Enable mesos to build against system installed version of protobuf.

Review: https://reviews.apache.org/r/23086
MESOS-1171: unbundle gmock

Enable support to build against unbundled versions of gmock.

Review: https://reviews.apache.org/r/24628
MESOS-1169: unbundle distribute

Enabling unbundling for distribute python utils.

Review: https://reviews.apache.org/r/24555
Added explicit DockerInfo within ContainerInfo.

Added new DockerInfo to explicitly capture Docker options, and allow
command URIs to be fetched and mapped into sandbox, which gets
bind-mounted into the container.

Review: https://reviews.apache.org/r/24475
Used new CommandInfo with Docker::run.

Review: https://reviews.apache.org/r/24673
Fix for configure --disable-python
MESOS-1693: enable build against non-x86 arch

Enable building against non-x86 arch, namely arm builders on Fedora.

Review: https://reviews.apache.org/r/24563
Fixed a few compiler warnings in stout.
Redirect Docker logs.
Added a user doc for framework rate limiting.

Review: https://reviews.apache.org/r/24667
Fixed missing '+' in docker.cpp.
Fixed style issues in slave/flags.hpp.
Allow override to run in executor