Pluggable Database/Message Broker/Distributed Coordination Services

OpenContrail comes with a set of external infrastructure dependencies, such as a database (Cassandra), a message broker (RabbitMQ) and a distributed coordination service (ZooKeeper). Users thus need to accept indirect costs of running these services on premises (administration, maintenance, updates etc).

Many enterprise standardize on some infrastructure stack to cut these costs down. This is especially true for public clouds where a managed database and a message queue are usually provided. Thus, it makes sense for OpenContrail to re-use these infrastructure choices rather than enforcing its own.

The work is already undergoing to let users choose between two configuration databases: Cassandra and RDBMS (MySQL). The scope of this blueprint is to:
* Amend this work so more databases can be easily added in future
* Extend it to the two remaining dependencies, namely RabbitMQ and ZooKeeper.

Analytics is out of the scope of this blueprint. Analytics comes with its own set of the external dependencies (Kafka and Redis are major ones), but we feel it's feasible to see the shape of analytics by the end of OC 5.0 release cycle before doing any changes to it.