Reap previously launched logviewer tasks

[srikanth-viswanathan]

Reap previously launched logviewer tasks if mesos.logviewer.sidecar.enabled is now false

Fixes #239

[srikanth-viswanathan]

The only visible side effect of this change is that you'll now see this line every 5 minutes if the logviewers znode doesn't exist (as would be the case if, say, logviewers were never enabled):

2018-03-17 03:25:16.968 s.m.u.ZKClient [ERROR] org.apache.zookeeper.KeeperException$NoNodeException: KeeperErrorCode = NoNode for /storm-mesos/logviewers

Do you think this is a problem? I'm open to your suggestions.

[srikanth-viswanathan]

@JessicaLHartog Thanks for the review. I've pushed changes that address your comments.

P.S.: I had originally added spaces around lines of code to add visual separation for readability. I've since removed them after your comments, but I'm curious about the reason for your suggestion.