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>

doc/source/architecture.rst

@@ -384,7 +384,9 @@ following methods of the :ref:`Action <action_definition>` handler:
 
 -   **preconditions()**: this method will make sure that all conditions are met
     before executing the action (for example, it makes sure that an instance
-    still exists before trying to migrate it).
+    still exists before trying to migrate it). If certain predefined conditions
+    are found in this phase, the Action is set to **SKIPPED** state and will
+    not be executed.
 -   **execute()**: this method is what triggers real commands on other
     OpenStack services (such as Nova, ...) in order to change target resource
     state. If the action is successfully executed, a notification message is