Reap child processes gracefully if greenlet thread gets killed

If ProcessLauncher is run from within a thread we should be able to
call wait after the thread is killed to reap the child processes.
Catching this exception and then proceeding to the cleanup phase
accomplishes this.

Closes-Bug: #1282206
Change-Id: I9c863864c25c0478b720fc57cf0dccbe9bc9d9f6
Merge "Reap child processes gracefully if greenlet thread gets killed"