Auto Devops support for environment-specific custom domain

Description

Auto DevOps' use of base domains is awesome for getting started quickly. But what if you don't want your customers hitting production.foo.com as the only production name? This will be especially important for clusters shared between multiple projects (like at a group level) where the project name appears in the hostname as well.

Proposal

• Let users provide a custom domain, not just a base domain, for a specific environment for project.
• This could leverage environment-specific variables for tiers that offer it, but for Core users, perhaps <env>_DOMAIN should work too.

Notes/Questions

1. I'm not sure if we need to validate ownership of the domain first, like we do for Pages.

Links / references

Possible workaround in https://gitlab.com/gitlab-org/gitlab-ce/issues/46954#note_128377297