manager: fix ImportError in scheduler.py

circle/manager/scheduler.py

-from vm.models import Node
-
 def get_node(instance):
 def get_node(instance):
     ''' Select a node for an hosting an instance
     ''' Select a node for an hosting an instance
     based on requirements.
     based on requirements.
     '''
     '''
     # Return first Node or None
     # Return first Node or None
+    from vm.models import Node
     try:
     try:
         return Node.objects.all()[0]
         return Node.objects.all()[0]
     except:
     except: