Support multiple email-sending domains per assessment

[dav3r]

💡 Summary

Enable an assessment to have more than one email-sending domain.

Motivation and context

Some types of assessments require multiple instances (e.g. Teamservers), where each instance sends email from a different domain. Currently, our email_sending_domain variable only supports a single domain.

Implementation notes

Convert the email_sending_domain variable from a string to a list.

Acceptance criteria

• [ ] Terraform and cloud-init function correctly with no email-sending domains, a single Teamserver, and a single Gophish instance
• [ ] Terraform and cloud-init function correctly with a single email-sending domain, a single Teamserver, and a single Gophish instance
• [ ] Terraform and cloud-init function correctly with two email-sending domains, two Teamservers, and two Gophish instances
• [ ] Documentation explains how list of email-sending domains is applied to instances that send mail (teamserver, gophish).

[jsf9k]

As part of this work, please update the description for the email_sending_domain variable to make it clear that the FQDN is expected.

[dav3r]

According to @kyleevers, multiple email-sending domains are almost never used in a single RPT environment, so implementing this feature is not as important as previously thought.