grid: always check ownership of software path before building

When reinstalling a computer, 'node software' may run before
'node format' has created users, then it fails because it can't set
ownership of the directory it created. The directory already exists
on next runs and before this commit, it didn't fix the ownership.

slapos/grid/SlapObject.py

@@ -251,10 +251,14 @@ class Software(object):
     root_stat = os.stat(self.software_root)
     os.environ = getCleanEnvironment(logger=self.logger,
                                      home_path=pwd.getpwuid(root_stat.st_uid).pw_dir)
-    if not os.path.isdir(self.software_path):
+    try:
       os.mkdir(self.software_path)
-      self._set_ownership(self.software_path)
+    except OSError as e:
+      if e.errno != errno.EEXIST:
+        raise
+    else:
       os.chmod(self.software_path, 0o755)
+    self._set_ownership(self.software_path)
 
     extends_cache = tempfile.mkdtemp()
     self._set_ownership(extends_cache)