Changes domain field to be on the Cluster page show, removing it from
Auto DevOps setting. Also injects the new environment variable
KUBE_INGRESS_BASE_DOMAIN into kubernetes#predefined_variables.

Migration to move the information from ProjectAutoDevops#domain
to Clusters::Cluster#domain. As well as necessary modifications to qa
selectors

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52363