Nimbus should kill already-launched logviewers if relaunched with `sidecar.enabled=false`

[srikanth-viswanathan]

Nimbus should kill already-launched logviewers if re-launched with mesos.logviewer.sidecar.enabled=false.

The pertinent use case is: If Nimbus is initially launched with mesos.logviewer.sidecar.enabled=true (or absent, which will give it the default true value) and then re-launched with mesos.logviewer.sidecar.enabled=false. logviewers launched prior to the re-launch are never reaped. Nimbus should reap them and release the resources back to Mesos.