cassandra v2 support

Adds support for Cassandra v2, and for Cassandra 

Note I'm having problems with `JAVA_HOME` which really should be addressed before merging.

In `CassandraDatacenterLiveTest.testDatacenterWithVnodesVersion2` (running on CentOS on EC2), I had to add `.configure("shell.env", MutableMap.of("JAVA_HOME", "/etc/alternatives/java_sdk_1.7.0"))`. The VM had Java 1.6 installed already; brooklyn then installed `java-1.7.0-openjdk-devel`. At that point, `which java` reported 1.7. However, running Cassandra (with the remote `ssh` commands) still picked up 1.6 until I explicitly set `JAVA_HOME`. This could perhaps be an issue with `~/.profile` etc not being sourced for the `ssh` commands.

@ahgittin @grkvlt any opinions on how to portably/reliably upgrade to Java 1.7 such that Cassandra picks it up?
Javadoc stating behaviour on null val would be a nice-to-have (ditto other methods in class)