Make default Planner generic to handle third-party action plugins

Watcher provides a very modular architecture that currently allows anyone to provide custom plugins:

- Strategies
- Planner
- Action
- Workflow Engine

In most of the above, their related implementation(s) are generic enough so that they can work with any 3rd party plugin out of the box. However, this is currently not the case for the "default" planner which defines a hardcoded set of priorities for each action type. Hence, this means that adding a new action_type via an action plugin and reference it in a strategy would result in an error and the only solution is to create a planner plugin that would adapt to this new action.