Follow up changes for skip-action blueprint

These are some of the requested changes from reviews
in the series of patches for add-skip-action blueprint.
Some of them may required another specific patch since
would touch in more files that are not related to
this feature.

Change-Id: I9e30ca385e7b184ab19449a60db6f6d0f3c0e1b9
Signed-off-by: Douglas Viroel <viroel@gmail.com>

doc/source/architecture.rst

@@ -384,8 +384,8 @@ 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). If certain predefined conditions
-    are found in this phase, the Action is set to **SKIPPED** state and will
+    still exists before trying to migrate it). If action specific preconditions
+    are not met 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