Add action description

Add action get_description in class BaseAction.
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Partially Implements: blueprint dynamic-action-description

Change-Id: I9ce1b18ad8c5eb7db62ec926d1859d0f508074b0

watcher/applier/actions/migration.py

@@ -172,3 +172,7 @@ class Migrate(base.BaseAction):
     def post_condition(self):
         # TODO(jed): check extra parameters (network response, etc.)
         pass
+
+    def get_description(self):
+        """Description of the action"""
+        return "Moving a VM instance from source_node to destination_node"