testcase: retry `slapos node report`

This command sometimes fail, for example with:

  2019-10-23 06:10:56 slapos[27556] ERROR
  Traceback (most recent call last):
    File "slapos/grid/SlapObject.py", line 800, in destroy
      self.cleanupFolder(self.instance_path)
    File "slapos/grid/SlapObject.py", line 836, in cleanupFolder
      os.remove(os.path.join(folder_path, file))
  FileNotFoundError: [Errno 2] No such file or directory: 'tmp/inst/defp5/.installed.cfg.tmp'

slapos/testing/testcase.py

@@ -260,7 +260,7 @@ class SlapOSInstanceTestCase(unittest.TestCase):
   # maximum retries for `slapos node instance`
   instance_max_retry = 10
   # maximum retries for `slapos node report`
-  report_max_retry = 0
+  report_max_retry = 2
   # number of partitions needed for this instance
   partition_count = 10
   # reference of the default requested partition