Cassandra DB support for MetaModel [METAMODEL-18]

Cassandra DB support using the datastax java driver, see: https://github.com/datastax/java-driver

Integration tests have been also added. These tests need a cassandra instance running, properties to enable the integration test for this module have been added to the example-metamodel-integrationtest.configuration.properties file.
Another good point. I have also modified the createElasticSearchDataContext that was returning also a QueryPostprocessDataContext