[muxcable][config] add CLI support for mux mode detach (#2425)

What I did
Add support for config mux mode detach. Stemming from sonic-net/sonic-linkmgrd#79

sign-off: Jing Zhang zhangjing@microsoft.com

How I did it
Add detach to the choice list.

How to verify it
Existing unit test.
Tested on DUT

config/muxcable.py

@@ -280,7 +280,7 @@ def update_configdb_pck_loss_data(config_db, port, val):
 
 # 'muxcable' command ("config muxcable mode <port|all> active|auto")
 @muxcable.command()
-@click.argument('state', metavar='<operation_status>', required=True, type=click.Choice(["active", "auto", "manual", "standby"]))
+@click.argument('state', metavar='<operation_status>', required=True, type=click.Choice(["active", "auto", "manual", "standby", "detach"]))
 @click.argument('port', metavar='<port_name>', required=True, default=None)
 @click.option('--json', 'json_output', required=False, is_flag=True, type=click.BOOL)
 @clicommon.pass_db