Have Runner join runner clients with timeout

[imcdo]

In the case that some client processes fail to join with the runner process after completion; this can cause all remaining test to fail as the runner is no longer recieving messages and can cause infinite waiting of a thread causing ducktape to not terminate. To address this, we can have the process be polled until completion, and terminate it if it failes to join in the correct amount of time.

[cla-assistant[bot]]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}