set up SPLUNK_HOME and SPLUNKFORWARDER_WEB_PORT

use var SPLUNK_HOME to replace "/opt/splunkforwarder"
setup var SPLUNKFORWARDER_WEB_PORT,so that we can change splunk web port easily

+1 to what @e0d  is saying.  I think it's better to pull in the file as a template.  That is the standard way we are updating configuration in other roles and I would like to keep using that unless there is a good reason not to.