The --username flag in grr_config_updater has been removed

It appears that --username is no longer a flag in **grr_config_updater**: 

```
sudo grr_config_updater add_user --username XXXXXX
usage: grr_config_updater [-h] [--verbose] [--debug] [--config CONFIG]
                          [--secondary_configs SECONDARY_CONFIGS]
                          [--config_help] [--context CONTEXT]
                          [--plugins PLUGINS] [-p PARAMETER] [--list_storage]
                          [--share_dir SHARE_DIR]
                          
                          {load_memory_drivers,generate_keys,repack_clients,initialize,update_user,add_user,delete_user,show_user,upload_raw,upload_artifact,upload_python,upload_exe,upload_memory_driver}
                          ...
grr_config_updater: error: unrecognized arguments: --username
```

Omitting the flag works just fine: 

```
sudo grr_config_updater add_user XXXXX
```