Mistral DSL

We need Mistral to support a DSL for defining task graphs. DSL should be YAML based and allow to:
- Define tasks (name, description)
- Define a list of tags associated with tasks
- Define dependencies between tasks (specifying task names or task tags)
- Define actions associated with tasks
- Define task revert actions
- Define task graph actions that tasks can be associated with (specifying transport)
- Define action transports (specifying transport type and details specific to this type, e.g. AMQP exchange, host, port etc.)
- Define workflow execution schedule (specifying a target task and cron pattern or specific time)