Provide a more dynamic Actions management solution

In Watcher, an Audit generates a set of actions which aims at achieving a given goal (lower energy consumption, ...). It is possible to configure different strategies in order to achieve each goal. Each strategy is written as a Python class which produces a set of actions.

Today, the set of possible actions is fixed for a given version of Watcher and enables optimization algorithms to include actions such as instance migration, changing hypervisor state, changing power state (ACPI level, ...).

It should be possible to deploy and configure (in watcher.conf) new potential actions and associate each action to a Python class, without the need to redeploy a new version of Watcher.
Each configured action should at least contain the following information:
- a unique id
- the target resource type (instance, compute node, ...) to which the action will apply
- a list of input parameters and for each parameter a type (UUID, Integer, Long, Float, Enum, ...) and a flag indicating whether it is mandatory or optional
- a list of output parameters which are returned by the action and may enable the Watcher Applier to update its current context for an Audit.