Simplify golang logic of hyphenated flags and env vars

[nitrocode]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

I'd like to add a new feature flag to atlantis without having to add it both as a --xyz flag and as an environment variable.

Describe the solution you'd like

Add it in a single place and have it create the flag and the env var.

This will result in less code and fewer contirbutions that forget to add it to one or the other.

See this PR on how to automate it aquasecurity/tfsec#1990

See this for inspiration https://github.com/carolynvs/stingoftheviper/blob/main/main.go

Flags

atlantis/cmd/server.go

Lines 36 to 41 in 1bd5cc2

	// To add a new flag you must:
	// 1. Add a const with the flag name (in alphabetic order).
	// 2. Add a new field to server.UserConfig and set the mapstructure tag equal to the flag name.
	// 3. Add your flag's description etc. to the stringFlags, intFlags, or boolFlags slices.
	const (
	// Flag names.

Env var

atlantis/server/user_config.go

Lines 10 to 13 in 1bd5cc2

	// UserConfig holds config values passed in by the user.
	// The mapstructure tags correspond to flags in cmd/server.go and are used when
	// the config is parsed from a YAML file.
	type UserConfig struct {

Describe the drawbacks of your solution

None

Describe alternatives you've considered

None