CDK-647: Update examples to use Flume's Log4jAppender and DatasetSink

 * This patch includes configuring the Log4jAppender to gracefully
    handle connection errors. This is useful as it avoids a circular
    dependency between loading the log4j configuration to create the
    dataset and starting Flume after the dataset exists.
 * The README for the logging, logging-webapp, and demo examples have
    been updated accordingly.
 * This patch also includes some fixes needed to work on the CDH5.1
    QuickStart VM. I suspect more of the examples may have issues.
    So I'll file a follow-up JIRA to check them out and to update the
    top-level README to account for changes between the 4.4 and 5.1 VMs.
Okay, that makes sense. So these docs links are only changed since we can't run anything from off-cluster anyway. I think that's pretty reasonable. If we change to requiring the commands to be run on a quickstart VM, then we could also switch over to using the dataset command.