autoscaler: fix double-prefixed worker log filename

Log file was autoscale-autoscale-1.log because worker_id already
carries the autoscale- prefix; now it is autoscale-1.log.

cloud/autoscaler.py

@@ -159,7 +159,7 @@ class Autoscaler:
         log_handle = None
         if self.log_dir:
             log_path = os.path.join(
-                self.log_dir, "autoscale-%s.log" % worker_id)
+                self.log_dir, "%s.log" % worker_id)
             log_handle = open(log_path, "ab")
         proc = subprocess.Popen(
             cmd, stdout=log_handle, stderr=subprocess.STDOUT,