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?
No good reason in my opinion - just makes the code harder to understand when there's no need for it to be dynamic. This code was already there, and is a bit fiddly to change because it's reusing `CassandraDatacenter.EXECUTE_SCRIPT` which reuses `DatastoreMixins.EXECUTE_SCRIPT`, both of which are built as "abstract" effectors.

Will leave this as-is for this PR.

I believe @ahgittin wrote this - Alex is keenest on dynamic effectors.

My view is they have their place when there is a need for it to be dynamic, but is just unnecessary indirection when a static `@Effector` annotation would do.