Reuse app domain for domain identity

[lorenyu]

Ticket

N/A

Changes

• Use service_config.domain_name for notifications-email-domain module
• DRY up usage of domain_name and hosted_zone_id in service root module by defining local variables

Context for reviewers

We originally thought that the domain for the SES domain identity we create had to be the same as the root of the hosted zone domain rather than a subdomain, but we were wrong. This change causes the domain identity to just reuse the same domain as the app's configured domain, which simplifies things and also removes the chance of conflict between two apps that enable notifications.

Moreover, we originally thought that using the root of the hosted domain is fine as long as there's only one app in a hosted zone that enabled notifications. However, if multiple environments of an app shares a hosted zone (e.g. on some projects multiple environments share a non-prod hosted zone) then there will be conflicts. This change also fixes those conflicts.

Testing

see 🔒 https://github.com/navapbc/pfml-starter-kit-app/pull/245