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 gives ability to create/delete instance
with cassandra datastire type. No other operations supported.

Change-Id: Ib48f6392765af49c2df1447c3df46675825d210f
Implements: blueprint cassandra-db-support
Add heat template for cassandra single instance

Reasons:
- trove could use heat as provisioning layer,
each new integration should come with heat template;
Changes:
- adding heat template

Change-Id: I5aa1207ce141558074f02c0225207d759ca554c6
Implements: blueprint cassandra-db-support