Salt configuration for fluentd to Elasticsearch

This is a starting point for discussion with a view to changing this salt fluentd configuration to be optional and enabled with a flag during cluster creation time. @jbeda please can you have a look at this and please suggest the best way forward for me to change the config and other programs (kubeup.sh?) to make the fluentd to Elasticsearch optional? Thank you kindly.
You don't want to use the `${FOO=bar}` construct as that'll have the non-obvious side effect of setting `$FOO`.

This line should probably read:
```
echo "readonly FLUENTD_ELASTICSEARCH='${FLUENTD_ELASTICSEARCH:-true}'"
```