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.
I haven't tried using HADOOP_CONF_DIR yet, but as you point out that doesn't help the Maven plugin, which we are currently using in the demo.

I started looking at hostname parameterization here: https://issues.cloudera.org/browse/CDK-660, however I like the idea of having the Maven plugin (optionally) be able to pick up environment variables. So that's something I think we should look into.

It's unfortunate that the latest QuickStart VM doesn't have CM enabled. We could just provide instructions on how to start CM - I think it's just a question of clicking on an icon on the desktop.