grid: output text for promise error when using SR python

Because we were reading subprocess output as bytes, the output in case
of promise errors was something like b'ERROR ...'

There was another problem writing string to an binary file in error path.

slapos/grid/promise/runpromises.py

@@ -64,5 +64,9 @@ os.dup2(2, 1)
 try:
   promise_checker.run()
 except Exception as e:
-  os.write(out, str(e))
+  if sys.version_info < (3,):
+    error_str = unicode(str(e), 'utf-8', 'repr')
+  else:
+    error_str = str(e)
+  os.write(out, error_str.encode('utf-8', 'repr'))
   sys.exit(2 if isinstance(e, PromiseError) else 1)

slapos/grid/slapgrid.py

@@ -730,6 +730,7 @@ stderr_logfile_backups=1
         command,
         preexec_fn=lambda: dropPrivileges(uid, gid, logger=self.logger),
         cwd=instance_path,
+        universal_newlines=True,
         stdout=subprocess.PIPE)
       promises = plugins + len(listifdir(legacy_promise_dir))
       # Add a timeout margin to let the process kill the promises and cleanup

slapos/tests/test_slapgrid.py

@@ -4173,6 +4173,26 @@ class TestSlapgridPluginPromiseWithInstancePython(TestSlapgridPromiseWithMaster)
       super(TestSlapgridPluginPromiseWithInstancePython, self).tearDown()
     self.assertEqual(self.expect_plugin, called)
 
+  def test_failed_promise_output(self):
+    computer = ComputerForTest(self.software_root, self.instance_root, 1, 1)
+    instance, = computer.instance_list
+
+    instance.requested_state = 'started'
+    instance.setPluginPromise(
+        "failing_promise_plugin.py",
+        promise_content="""if 1:
+          return self.logger.error("héhé fake promise plugin error")
+        """,
+    )
+
+    with httmock.HTTMock(computer.request_handler), \
+        patch.object(self.grid.logger, 'info',) as dummyLogger:
+      self.launchSlapgrid()
+
+    self.assertEqual(
+        dummyLogger.mock_calls[-1][1][0] % dummyLogger.mock_calls[-1][1][1:],
+        "  0[(not ready)]: Promise 'failing_promise_plugin.py' failed with output: héhé fake promise plugin error")
+
 
 class TestSVCBackend(unittest.TestCase):
   """Tests for supervisor backend.