Add configuration of Gitaly and associated secrets to migrations chart.
Some migrations require access to Gitaly, meaning that we now need these
present within the pod.

Adds
- `.Values.gitaly` template
- ConfigMap
  - move to `gitlab.yml.erb`
  - add `required` configuration of gitaly to `gitlab.scripts.configure`
  - trim `optional` from `gitlab.scripts.configure`
- Update job with necessary mount template for Gitaly.