Document how to use a static ip for GKE clusters
Related to: https://gitlab.com/gitlab-org/gitlab-ce/issues/42760
By default, an ephemeral external address is associated to a load balancer, but we want users to be aware they have to turn into a static reserved one if they rely on this ip (e.g., long living projects).
Documentation should report this scenario, and explain how to promote the automatically assigned ephemeral address for clusters on GKE.
Edited by Fabio Busatto