Added default value to CPU usage and memory usage

If the monitor info can't get the values, the values get default 0.

circle/vm/models/node.py

@@ -366,12 +366,18 @@ class Node(OperatedMixin, TimeStampedModel):
     @property
     @node_available
     def cpu_usage(self):
+        if self.monitor_info.get('cpu.percent') is not None:
             return self.monitor_info.get('cpu.percent') / 100
+        else:
+            return 0
 
     @property
     @node_available
     def ram_usage(self):
+        if self.monitor_info.get('memory.usage') is not None:
             return self.monitor_info.get('memory.usage') / 100
+        else:
+            return 0
 
     @property
     @node_available