add mustEnv function

[C-Pro]

Background

It is important to fail early and not try to deploy a config with wrong data. But when populating values using env function a typo in variable name will lead to an empty value instead of an error, like when trying to access nonexistent vault key with error_on_missing_key enabled.

This PR addresses it with adding extra mustEnv command that fails if environment variable does not exist or is empty.

$ cat env.tmpl
URL={{mustEnv "ENV"}}:12312

$ ENV=test ~/go/bin/consul-template -once -template=env.tmpl -dry
>
URL=test:12312

$ ~/go/bin/consul-template -once -template=env.tmpl -dry
2022-10-20T18:21:52.794+0700 [ERR] (cli) env.tmpl: execute: template: :1:6: executing "" at <mustEnv "ENV">: error calling mustEnv: required environment variable ENV is empty

[hashicorp-cla]

All committers have signed the CLA.

[eikenb]

code, docs, tests... 👍