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 filed [CDK-656](https://issues.cloudera.org/browse/CDK-656) to go through and update the hostname in all of the examples and to also update the docs.

@tomwhite, have you had a chance to try out the new support for setting HADOOP_CONF_DIR and HIVE_CONF_DIR so that the examples pick up the configuration from the environment? The only thing that doesn't work for is the maven plugin.

For the maven plugin, and potentially the default config files that we ship, we could make the hostnames changeable at run time by pulling them from a maven property and filtering the resources when we copy. This would require the user to add something like `-D hadoop.namenode.host=nn.cluster.com` on the Maven commandline. I don't like it as much as the environment variable based configuration because you have to remember it on every command rather than setting the variables once, but unless we can update the maven plugin to get it's configuration from something other than the pom, we may be stuck.

One final point. In my experience, the way that users do remote development and testing against a cluster is to download the client configuration files from CM and either placing them into their $HADOOP_HOME/etc/hadoop or setting HADOOP_CONF_DIR. The problem with recommending that for use with the QuickStart VM is that CM isn't enabled by default, so you'd have to SSH in to pull the config files. Unfortunately, there is no port forwarding configured to allow SSH, so we'd have to provide instructions for that.