Fix typo for create_config_drive script

Change-Id: I6fb32dbceeb85545bc37888f4df0f25323edc071
Related-Prod: PROD-28144 (PROD:28144)
diff --git a/config-drive/create_config_drive.py b/config-drive/create_config_drive.py
index 55b96e3..4a0671e 100755
--- a/config-drive/create_config_drive.py
+++ b/config-drive/create_config_drive.py
@@ -154,7 +154,7 @@
         if username:
             with open(user_file_path, 'a') as user_file:
                 users_data = "#cloud-config\n"
-                users_data = "users:\n"
+                users_data += "users:\n"
                 users_data += "  - name: %s\n" % username
                 users_data += "    sudo: ALL=(ALL) NOPASSWD:ALL\n"
                 users_data += "    groups: admin\n"