Add action description

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

Change-Id: I9ce1b18ad8c5eb7db62ec926d1859d0f508074b0

watcher/tests/applier/workflow_engine/test_default_workflow_engine.py

@@ -52,6 +52,9 @@ class FakeAction(abase.BaseAction):
     def execute(self):
         raise ExpectedException()
 
+    def get_description(self):
+        return "fake action, just for test"
+
 
 class TestDefaultWorkFlowEngine(base.DbTestCase):
     def setUp(self):