slap/standalone: Provide well known configuration file to supervisord

The supervisord binary in some environments (like webrunner), seems to not
follow the automatic way to find the configuration file, so instead of relying
on it, just explicitly provide the file.

/reviewed-on nexedi/slapos.core!168

slapos/slap/standalone.py

@@ -704,7 +704,7 @@ class StandaloneSlapOS(object):
           self._logger.debug("Supervisor pid file seem stale")
     # start new supervisord
     output = subprocess.check_output(
-        ['supervisord'],
+        ['supervisord', '--configuration', self._supervisor_config],
         cwd=self._base_directory,
     )
     self._logger.debug("Started new supervisor: %s", output)