Launch subprocesses associated with specified namespaces.

Currently there is no standard way in Mesos to launch a child process in a different namespace (e.g. net, mnt). A user may leverage Subprocess and provide its own clone callback, but this approach is error-prone.

One possible solution is to implement a Subprocess' child hook. In MESOS-5070, we have introduced a child hook framework in subprocess and implemented three child hooks CHDIR, SETSID and SUPERVISOR. We suggest to introduce another child hook SETNS so that other components (e.g., health check) can call it to enter the namespaces of a specific process.