explicitly specify command with underscores

Starting click 7.0. The default behavior of a command with under
scores will be replace with dashes.

this is to address the above default behavior change, so that
the command remains the same.

more details can be found:

pallets/click#1123

config/main.py

@@ -641,7 +641,7 @@ def reload(filename, yes, load_sysinfo):
     log_info("'reload' restarting services...")
     _restart_services()
 
-@config.command()
+@config.command("load_mgmt_config")
 @click.option('-y', '--yes', is_flag=True, callback=_abort_if_false,
                 expose_value=False, prompt='Reload mgmt config?')
 @click.argument('filename', default='/etc/sonic/device_desc.xml', type=click.Path(exists=True))
@@ -665,7 +665,7 @@ def load_mgmt_config(filename):
     run_command(command, display_cmd=True, ignore_error=True)
     click.echo("Please note loaded setting will be lost after system reboot. To preserve setting, run `config save`.")
 
-@config.command()
+@config.command("load_minigraph")
 @click.option('-y', '--yes', is_flag=True, callback=_abort_if_false,
                 expose_value=False, prompt='Reload config from minigraph?')
 def load_minigraph():

pfcwd/main.py

@@ -221,7 +221,7 @@ def stop(ports):
         configdb.mod_entry(CONFIG_DB_PFC_WD_TABLE_NAME, port, None)
 
 # Set WD default configuration on server facing ports when enable flag is on
-@cli.command()
+@cli.command("start_default")
 def start_default():
     """ Start PFC WD by default configurations  """
     if os.geteuid() != 0: