'get_volume_type_by_backendname' returns a list

Storage pool can have many volume types,
'get_volume_type_by_backendname' should return a list of types.

Closes-Bug: #1733257
Change-Id: I877d5886259e482089ed0f9944d97bb99f375824

watcher/decision_engine/model/element/node.py

@@ -56,7 +56,7 @@ class StorageNode(storage_resource.StorageResource):
         "zone": wfields.StringField(),
         "status": wfields.StringField(default=ServiceState.ENABLED.value),
         "state": wfields.StringField(default=ServiceState.ONLINE.value),
-        "volume_type": wfields.StringField()
+        "volume_type": wfields.ListOfStringsField()
     }
 
     def accept(self, visitor):

watcher/decision_engine/model/model_root.py

@@ -508,7 +508,13 @@ class StorageModelRoot(nx.DiGraph, base.Model):
 
         root = etree.fromstring(data)
         for cn in root.findall('.//StorageNode'):
-            node = element.StorageNode(**cn.attrib)
+            ndata = {}
+            for attr, val in cn.items():
+                ndata[attr] = val
+            volume_type = ndata.get('volume_type')
+            if volume_type:
+                ndata['volume_type'] = [volume_type]
+            node = element.StorageNode(**ndata)
             model.add_node(node)
 
         for p in root.findall('.//Pool'):