storage: fix _atomic_write to create hidden temporary files (#2010)

wcs/qommon/storage.py

@@ -51,7 +51,9 @@ def atomic_write(path, content, async=False):
     '''Rewrite a complete file automatically, that is write to new file with
     temporary name, fsync, then rename to final name. Use threads to remove blocking.'''
     def doit():
-        fd, temp = tempfile.mkstemp(dir=os.path.dirname(path), prefix=os.path.basename(path))
+        dirname = os.path.dirname(path)
+        fd, temp = tempfile.mkstemp(dir=dirname,
+                prefix='.tmp-'+os.path.basename(path)+'-')
         f = os.fdopen(fd, "w")
         if hasattr(content, 'read'):
             # file pointer