Previously, we did not attempt to stop Docker Logger processes until
DestroyTask, which means that under many circumstances, we will never
successfully close the plugin client.

This commit terminates the plugin process when `run` terminates, or when
`DestroyTask` is called.

Steps to repro:

```
$ nomad agent -dev
$ nomad init
$ nomad run example.nomad
$ nomad stop example
$ ps aux | grep nomad # See docker logger process running
$ signal the dev agent
$ ps aux | grep nomad # See docker logger process running
```