[Analytics] Use a classloader to isolate in-jvm dtest classes in integration test

We need to isolate the in-jvm dtest classes (coming from the shaded dtest jar) from the classes in Cassandra Analytics when running integration tests. The isolation will allow us to test the Analytics code in the AppClassloader without any class pollution from classes coming from the shaded dtest jar. In some cases, the pollution can spill to classes that are shaded in the analytics project (metrics, json serialization, guava, etc).

With the isolation in place we can be more confident that the code we are testing is the code that will be running in a Spark environment.