Allow override of data, log and naming configs

* Allow override of data directory
* Allow override of log directory
* Allow specification of cluster name
* Allow specification of node name, default to hostname
At the moment, the default should remain the same as it is in all the ES deployments. There is no reason to move it to /data until /data is a dedicated LV. 

If I change it now, it makes for a really horrid migration to this version of the formula.