Support specifying kong-admin-token using an environment variable #1730
Description
The --kong-addr can be specified using an environment variable DECK_KONG_ADDR, this is very good and it saves a lot of time when executing deck command multiple times.
If you can also support using another environment variable, for example DECK_KONG_ADMIN_TOKEN, to specify the admin token, that is even better.
It is not only more convenient, it's more secure. Because when people share their steps, they copy commands: by only specifying the admin token once, they hide the token from subsequent commands.
