added option "ipprefix"

This is the first of two patches to easily switch to another ipspace while running the tests for java-driver.
The other half (java-driver) is under https://github.com/damoon/java-driver/tree/ipprefix.
In combination it enables developing/jenkins-building the java-driver without shutting down an already running cassandra instance at 127.0.0.1.

* the ipprefix is "127.0.0." by default
* nodeips get generated in this ipspace while populating
* it is an optional parameter for create and populate

It is passing all tests for java-driver in the new ipspace on my local laptop.