[scheduler] Fix possible failure during first object task launch

This issue may happen during the first task launch on an object when some other tasks are also running on this object.
We have then following node.scheduler.log:
 failed run '%s' exit code:1"

This patch fix following silent stack

    Traceback (most recent call last):
      File /usr/lib/python3.8/runpy.py, line 194, in _run_module_as_main
        return _run_code(code, main_globals, None,
      File /usr/lib/python3.8/runpy.py, line 87, in _run_code
        exec(code, run_globals)
      File /usr/share/opensvc/opensvc/__main__.py, line 131, in <module>
        ret = main(sys.argv)
      File /usr/share/opensvc/opensvc/__main__.py, line 74, in main
        ret = Mgr()(argv=argv[2:])
      File /usr/share/opensvc/opensvc/commands/mgr/__init__.py, line 357, in __call__
        ret = self._main(argv=argv)
      File /usr/share/opensvc/opensvc/commands/mgr/__init__.py, line 331, in _main
        ret = self.do_svcs_action(options, action, argv=argv)
      File /usr/share/opensvc/opensvc/commands/mgr/__init__.py, line 137, in do_svcs_action
        ret = self.node.do_svcs_action(action, options)
      File /usr/share/opensvc/opensvc/core/node/node.py, line 2452, in do_svcs_action
        data, err, errs = self.do_svcs_action_sequential(action, options, svcs, need_aggregate)
      File /usr/share/opensvc/opensvc/core/node/node.py, line 2491, in do_svcs_action_sequential
        ret = svc.action(action, options)
      File /usr/share/opensvc/opensvc/core/objects/svc.py, line 925, in action
        return self._action(action, options=options)
      File /usr/share/opensvc/opensvc/core/scheduler.py, line 166, in _func
        self.sched.action_timestamps(action, options.rid)
      File /usr/share/opensvc/opensvc/core/scheduler.py, line 1053, in action_timestamps
        timestamp(tsfile, last=last)
      File /usr/share/opensvc/opensvc/core/scheduler.py, line 150, in timestamp
        os.makedirs(timestamp_d, 0o755)
      File /usr/lib/python3.8/os.py, line 223, in makedirs
        mkdir(name, mode)
    FileExistsError: [Errno 17] File exists: '/var/lib/opensvc/svc/<svcname>/scheduler'

opensvc/core/scheduler.py

@@ -145,12 +145,20 @@ def timestamp(timestamp_f, last=None):
         last = time.time()
     elif type(last) == datetime.datetime:
         last = to_time(last)
-    timestamp_d = os.path.dirname(timestamp_f)
-    if not os.path.isdir(timestamp_d):
-        os.makedirs(timestamp_d, 0o755)
     buff = repr(last)
-    with open(timestamp_f, 'w') as ofile:
-        ofile.write(buff+os.linesep)
+    try:
+        with open(timestamp_f, 'w') as ofile:
+            ofile.write(buff+os.linesep)
+    except (OSError, IOError):
+        timestamp_d = os.path.dirname(timestamp_f)
+        try:
+            os.makedirs(timestamp_d, 0o755)
+        except (OSError, IOError):
+            # another // call may have created this dir
+            # ignore error, and try again write file
+            pass
+        with open(timestamp_f, 'w') as ofile:
+            ofile.write(buff+os.linesep)
 
 
 def sched_action(func):