explicitly specify command with underscores

[lguohan]

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

- What I did

- How I did it

- How to verify it
tested for under current click and click 7.0

under buster

admin@vlab-01:/$ sudo config load
load              load_mgmt_config  load_minigraph    
admin@vlab-01:/$ sudo config load

- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)

[jleveque]

I think moving forward we should create all new subcommands using hyphens, and eventually transition these to use hyphens also. We can transition by using aliases for a few versions.

[jleveque]

@lguohan: I'm not sure if you want to handle other CLI utilities in this PR also, but I know for a fact sonic_installer has many subcommands which contain underscores.

[lguohan]

@jleveque , i agree with you.