When the Docker driver kills as task, we send a request via the Docker API for
dockerd to fire the signal. We send that signal and then block for the
`kill_timeout` waiting for the container to exit. But if the Docker API
blocks, we will block indefinitely because we haven't configured the API call
with the same timeout.

This changeset is a minimal intervention to add the timeout to the Docker API
call _only_ when we have the `kill_timeout` set. Future work should examine
whether we should be threading contexts through other `go-dockerclient` API
calls.