Support CLASSPATH when initializing ZooKeeper for tests

On certain setups (for example on vanilla Debian squeeze with system packages) the testing with ZooKeeper cluster fails in not finding the log4j1.2, even though that is installed under `/usr/share/java`. Reason is the code searches for `lib/*.jar` and does not include jar files in `ZOOKEEPER_HOME`.

However, instead of changing or adding more jars to the mix, this adds support for defining `CLASSPATH` as environment value and respecting it when constructing the classpath for `-cp`