Allow wait_for to wait on non-traditional files

Use os.path.exists to check for file existence, instead of "can we open
this file for reading".

Fixes #6710

library/utilities/wait_for

@@ -176,21 +176,24 @@ def main():
         end = start + datetime.timedelta(seconds=timeout)
         while datetime.datetime.now() < end:
             if path:
+                if os.path.exists(path):
+                    if search_regex:
                         try:
                             f = open(path)
                             try:
-                        if search_regex:
                                 if re.search(search_regex, f.read(), re.MULTILINE):
                                     break
                                 else:
                                     time.sleep(1)
-                        else:
-                            break
                             finally:
                                 f.close()
                         except IOError:
                             time.sleep(1)
                             pass
+                    else:
+                        break
+                else:
+                    time.sleep(1)
             elif port:
                 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
                 s.settimeout(connect_timeout)