Added NOMAD_HTTP_AUTH env var for basic auth

[kyhavlov]

Followed the model used by Consul as mentioned by @dadgar in #1416.

[dadgar]

LGTM! Thanks!

[sheerun]

@kyhavlov For what servers client connects to NOMAD_HTTP_AUTH is used for? What if client lists 3 servers? I don't fully understand this configuration option.

[schmichael]

For what servers client connects to NOMAD_HTTP_AUTH is used for? What if client lists 3 servers? I don't fully understand this configuration option.

The main use case is for people who run proxies between their CLI and HTTP API. This is a useful pattern for clusters with mTLS enabled as the proxy can handle mTLS internally and externally use a publicly signed certificate such as from Let's Encrypt.

[github-actions]

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