Use thread name in launcher ID

The Thread.ident attribute is apparently returning None for some
reason in tests, causing our launchers to share an ID. Change it
to Thread.name, which we know has a value.

Change-Id: Ib5c54804224ffd147372b2d1cfa6b62cc8a8b4cc

nodepool/nodepool.py

@@ -1021,7 +1021,7 @@ class ProviderWorker(threading.Thread):
         self.zk = self.getZK()
         self.launcher_id = "%s-%s-%s" % (socket.gethostname(),
                                          os.getpid(),
-                                         self.ident)
+                                         self.name)
 
     #----------------------------------------------------------------
     # Private methods