Add extra safety belt when reusing a node

When reusing a node we lock it but don't check if the state is still
ready. There are probably few to no possibilities how this can happen.
But it's better to guarantee this condition than to think that this
should be ok.

Change-Id: I365de6accb28b538dae5ff1dfb6c4a9966e50b01

nodepool/driver/__init__.py

@@ -437,6 +437,11 @@ class NodeRequestHandler(NodeRequestHandlerNotifications,
                         # It's already locked so skip it.
                         continue
                     else:
+                        # Add an extra safety check that the node is still
+                        # ready.
+                        if node.state != zk.READY:
+                            self.zk.unlockNode(node)
+                            continue
                         if self.paused:
                             self.log.debug("Unpaused request %s", self.request)
                             self.paused = False