Enable overriding of graphite root path prefix

This patch enables one to set a different datawriter in `gatling.conf` which can influence the graphite path
```
data {
		writers = [console, file, CustomGraphiteWriter ]
}
```
Example implementation:
```scala
class CustomGraphiteWriter extends MyDataWriter {
      
	override def getRootPathPrefix() : String = { 
		// val machinename = ?
		// val databasename= ?
		"loadtest.machinename.databasename" 
	}
}
```