Skip actions automatically based on pre_condition results

This patch is implementing skipping automatically actions based on the
result of action pre_condition method. This will allow to manage
properly situations as migration actions for vms which does not longer
exist. This patch includes:

- Adding a new state SKIPPED to the Action objects.
- Add a new Exception ActionSkipped. An action which raises it from the
  pre_condition execution is moved to SKIPPED state.
- pre_condition will not be executed for any action in SKIPPED state.
- execute will not be executed for any action in SKIPPED or FAILED state.
- post_condition will not be executed for any action in SKIPPED state.
- moving transition to ONGOING from pre_condition to execute. That means
  that actions raising ActionSkipped will move from PENDING to SKIPPED
  while actions raising any other Exception will move from PENDING to
  FAILED.
- Adding information on action failed or skipped state to the
  `status_message` field.
- Adding a new option to the testing action nop to simulate skipping on
  pre_condition, so that we can easily test it.

Implements: blueprint add-skip-actions

Assisted-By: Cursor (claude-4-sonnet)

Change-Id: I59cb4c7006c7c3bcc5ff2071886d3e2929800f9e
Signed-off-by: Alfredo Moralejo <amoralej@redhat.com>

api-ref/source/watcher-api-v1-actions.inc

@@ -23,6 +23,9 @@ following:
 
 -  **PENDING** : the ``Action`` has not been executed yet by the
    ``Watcher Applier``.
+-  **SKIPPED** : the ``Action`` will not be executed because a predefined
+   skipping condition is found by ``Watcher Applier`` or is explicitly
+   skipped by the ``Administrator``.
 -  **ONGOING** : the ``Action`` is currently being processed by the
    ``Watcher Applier``.
 -  **SUCCEEDED** : the ``Action`` has been executed successfully
@@ -152,4 +155,4 @@ Response
 **Example JSON representation of an Action:**
 
 .. literalinclude:: samples/actions-show-response.json
-   :language: javascript
+   :language: javascript