document quickstart config variables

[qrkourier]

The ziti create config and quickstart shell functions use env vars to configure things like TCP port and DNS name. One user reported the vars aren't the same for local vs. Docker Compose quickstarts, and they couldn't find a reference for the vars' meanings.

[qrkourier]

One way to introspect the config generator is to exec ziti create config environment. This prints the available option vars with descriptions by rendering an embedded template that responds to any vars that are already defined, and showing defaults for those that are now.