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.
We can change the URLs, but we're going to have to update the setup instructions to add quickstart.cloudera as an alias. The examples don't work properly if they have the NN configured to use localhost as the hostname, so the only way to run them from a host computer will be to add the alias on the host. Unless we get them to change the VM image back to using localhost.