agent/auto-auth: Add min_backoff to set first backoff value

[jasonodonnell]

This adds a new config to Agent's auto-auth.method's stanza: min_backoff.

Agent's auto-auth has an initial hardcoded retry duration of 1 second, which grows after each failed retry. I'm making this value configurable because there may be instances you want to retry faster/slower. While writing this feature I found that the SDK client used by auto-auth has it's own built in retry logic. I disabled that retry logic (set MaxRetries to 0) because the auto-auth code has its own backoff function and we were effectively doubling our retry attempts.

Templating also relies on auto-auth and has it's own retry logic. I found an instance where Vault failed to get a token but the templating server tried to fetch secrets. This resulted in yet another variation of retries and more requests to Vault. To combat this, I synced the Template server's initial backoff value to that of auto-auths.

Example config:

auto_auth = {
  method = {
    config = {
      role_id_file_path = "/tmp/agent/roleid"
      secret_id_file_path = "/tmp/agent/secretid"
    }

    type = "approle"
    min_backoff = "5s"
    max_backoff = "30s"
  }

  sink = {
    config = {
      path = "/tmp/file-foo"
    }
    type = "file"
  }
}

exit_after_auth = false

template = {
  contents = "{{ with secret \"secret/hashiconf\" }}{{ .Data | toJSONPretty }}{{ end }}"
  destination = "/tmp/agent/kv"
}

vault = {
  address = "http://127.0.0.1:8200"
}

[calvn]

One last doc comment, otherwise looks like it's good to go!