slapgrid: Ignore slapformat dump file in warning

Running slapformat creates a `.slapos-resource` file in each partition.
Running slapgrid emits a warning if a free partition dir is not empty.

With this commit, slapgrid only emits a warning if a free partition
contains entries other than the slapformat dump file.

slapos/grid/slapgrid.py

@@ -1307,7 +1307,8 @@ stderr_logfile_backups=1
               else:
                 shutil.rmtree(garbage_path)
 
-          if os.listdir(computer_partition_path) != []:
+          # Ignore .slapos-resource file dumped by slapformat.
+          if os.listdir(computer_partition_path) not in ([], ['.slapos-resource']):
             self.logger.warning("Free partition %s contains file(s) in %s." % (
                 computer_partition.getId(), computer_partition_path))
           continue