Introduce redis.announce_ip_from_hostname option, to enable setting the hostname at runtime

What does this MR do?

We recently introduced hostname support for redis+sentinel in !5853 (merged). However, it is currently not possible to dynamically set the hostname based on hostname -f. Thus, it requires statically configuring the hostname for each IP, which means per-node configuration in chef, as opposed to per-role. That is quite brittle.

This patch introduces a new setting redis['announce_ip_from_hostname'], which is mutually exclusive with redis['announce_ip'].

When enabled, it modifies the runit script to dynamically set --replica-announce-ip "$(hostname -f)".

Related issues

• gitlab-com/gl-infra/scalability#1661 (closed)

Checklist

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