URL configuration within secrets uses Route53 domains inconsistently

[richardTowers]

Many of our apps take environment variables such as DATABASE_URL, REDIS_URL etc. which specify several connection parameters, such as hostname, username and password.

These are provided by secretKeyRefs, which refer to k8s secrets.

The hostnames in the URLs inside these secrets is not consistent between apps or between environments. Some apps in some environments use URLs like signon-mysql.integration.govuk-internal.digital, whereas others use the automatically generated RDS hostnames.

There's a possibility this inconsistency could lead to someone making a mistake (e.g. thinking that updating an entry in Route53 would switch traffic between databases, when the apps aren't using that DNS entry).

Whether we go with Route53 hosts or autogenerated hosts, it would be nice to be consistent.