Fix possible race condition applying schemas

Found an instance where two of these fired too close together, and
caused a fatal error. Use a write lock, and then catch the schema.apply
function in case it fails anyways.

node-daemon/pvcnoded/Daemon.py

@@ -197,7 +197,9 @@ def entrypoint():
         os.execv(sys.argv[0], sys.argv)
 
     # Validate the schema
-    pvcnoded.util.zookeeper.validate_schema(logger, zkhandler)
+    with zkhandler.writelock("base.schema.version"):
+        sleep(0.5)
+        pvcnoded.util.zookeeper.validate_schema(logger, zkhandler)
 
     # Define a cleanup function
     def cleanup(failure=False):