slapgrid: Fix promise deadlock

Avoid use Popen.communicate instead of SlapPopen to run promises,
because SlapPopen uses threads and preexec_fn is not thread-safe.

The drawback is that logs are not propagated in real-time anymore,
but only once the promise process completes.

slapos/grid/slapgrid.py

@@ -739,17 +739,17 @@ stderr_logfile_backups=1
         dropPrivileges(uid, gid, logger=self.logger)
         os.dup2(err, 2)
       try:
-        process = SlapPopen(
+        process = subprocess.Popen(
           command,
           preexec_fn=preexec_fn,
           cwd=instance_path,
           universal_newlines=True,
           stdout=subprocess.PIPE,
           stderr=subprocess.PIPE,
-          logger=self.logger,
-          timeout=timeout,
         )
-        stderr = process.stderr.read()
+        stdout, stderr = process.communicate(timeout=timeout)
+        for line in stdout.splitlines():
+          self.logger.info(line)
         if process.returncode == 2:
           raise PromiseError(stderr)
         elif process.returncode: