Refactored Subprocess to support execve style launch and customized
clone function.

Review: https://reviews.apache.org/r/22831
Updated mesos to adapt to the new Subprocess interfaces.

Review: https://reviews.apache.org/r/22851
Introduced a Subcommand abstraction in stout.

Review: https://reviews.apache.org/r/22764/
Added subcommand tests to stout tests.

Review: https://reviews.apache.org/r/22764
Reverted Operation in favor of using Subcommand.

Review: https://reviews.apache.org/r/22764
Refactored the mesos containerizer launcher to fix MESOS-1404.

Review: https://reviews.apache.org/r/22852