Initial support for single instance Cassandra Database

Reasons
- this code adds ability to create/delete instance
with mongodb service type. No other operations supported.
- trove should support multiple extensions. One of the is NoSQL support.
Main goal is to add support for Apache Cassandra NoSQL database.

Changes:
- this code adds ability to create/delete instance
with cassandra service type. No other operations supported;
- adding datastore/version registration for cassandra

Change-Id: I67733f0ba18aae513465d66bb48e44bbb316a575
Implements: blueprint cassandra-db-support