Problem: Too many confusing variables, and scripts to run integration tests.

Solution:

• A bit of house keeping.
• Dropped the REUSE_KEYS environment variable. When a scenario is run and output.json is found, the generated keys will be loaded from it.
• Updated scripts to explicitly remove output.json where appropriate. eg. when scenario is run to initialize a new network.
• Adding a new scenario called flow which can be used to run an individual flow. This allows us to more flexibly run flows without need to group them in a scenario.
• Changed default behavior of yarn test and yarn start to start a test chain (TESTING runtime profile)