Include changes to isolation flag when creating Mesos containerizer.
Move Linux namespace functions into linux/.

Also moved and updated tests.

A following commit will remove this code from stout.

Review: https://reviews.apache.org/r/27091
Add getns() for namespaces.

Review: https://reviews.apache.org/r/27127
Add ns::pid::destroy() to destroy a pid namespace.

All processes are signalled with SIGKILL then reaped. The order of
signalling is not determined, i.e., generally the init pid is not the
first pid signalled.

Review: https://reviews.apache.org/r/25966/
Remove Linux namespace functions from stout.

This code was moved to linux/ in an earlier commit.

Review: https://reviews.apache.org/r/27092
Pid namespace isolator for the MesosContainerizer.

Add namespaces/pid to --isolation slave flag. Places executor into a pid
namespace so it and all descendants will be contained in the namespace.

Review: https://reviews.apache.org/r/25865
Correctly recover pid in Linux launcher.

If the freezer cgroup is absent (if the slave terminates after the
cgroup is destroyed but before realizing) we should still recover the
pid because we check this on destroy().

Review: https://reviews.apache.org/r/27166/
Use pid namespace to destroy container when available.

The Linux launcher will check if a container is running in a pid
namespace and will kill all processes rather than using the freezer.
This approach is backwards and forwards compatible.

Review: https://reviews.apache.org/r/25966