Add deployment update and deploy-webapp-from-registry commands. (#676)

stack_orchestrator/deploy/deployer.py

@@ -27,6 +27,10 @@ class Deployer(ABC):
     def down(self, timeout, volumes):
         pass
 
+    @abstractmethod
+    def update(self):
+        pass
+
     @abstractmethod
     def ps(self):
         pass