Add various env variables

Closed username-removed-90457 requested to merge pierreozoux/gitlab-ce:env_variables into master

What does this MR do?

It exposes more env variables.

Are there points in the code the reviewer needs to double check?

It would be my first commit, and I'm not really a rails developer. I'm not sure it doesn't collide with other initialization, but I did my best.

Why was this MR needed?

Working on docker, it is always better to have as much as possible env variables, it makes things a lot easier to configure. This MR would allow to remove this: https://gitlab.com/charts/charts.gitlab.io/blob/master/charts/gitlab-ce/templates/configmap.yaml#L17-51

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?