Add ntp_host config for gitaly cluster docs

What does this MR do?

The use of NTP_HOST and its default of pool.ntp.org was undocumented, and led to US Fed 4135 , where the customer's gitlab-rake gitlab:gitaly:check was failing with an I/o error reaching out to pool.ntp.org. A separate MR was opened to improve the error message.

Related issues

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