Rename API domain override environment variable

What does this MR do?

Renames the DAST_API_DOMAIN_OVERRIDE environment variable to DAST_API_HOST_OVERRIDE. This reflects the fact that both the HTTP domain name and port can be overridden.

This environment variable doesn't need to be deprecated as the base functionality hasn't been released.

What are the relevant issue numbers?

gitlab-org/gitlab#10928 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer