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