Allow for workflow-manager-api to talk to other DB instances for development & integration testing

The current behavior of workflow-manager is to use the AWS API to find the RDS instance that it should talk to. While very useful for production (the server need not be pre-configured with RDS specifics), it's more difficult to run an instance for development or integration testing.

This issue is for refactoring the server so that it can talk to:
1. An in-memory SQLite DB (similar to what we do for tests)
2. An in-cluster PostgreSQL DB (useful for integration tests or development)

This work should be done after #138 to prevent a very large merge conflict