To make etcd models object more like a dict.

Add items iteritems and keys functions in etcd.models:Base to
make the etcd model object more like a dict. When we need pass
the dict object, we don't need use model.as_dict any more, pass
the model object will be ok. Just like the way we use sql model
object.

Change-Id: Ic4d7e1c1cc266ee6d265b56e0ab56643ec86105a

zun/db/etcd/models.py

@@ -70,6 +70,18 @@ class Base(object):
         client.write(path, json.dump_as_bytes(self.as_dict()))
         return
 
+    def items(self):
+        """Make the model object behave like a dict."""
+        return self.as_dict().items()
+
+    def iteritems(self):
+        """Make the model object behave like a dict."""
+        return self.as_dict().items()
+
+    def keys(self):
+        """Make the model object behave like a dict."""
+        return [key for key, value in self.iteritems()]
+
 
 class ZunService(Base):
     """Represents health status of various zun services"""