Add global defaults for pull policy and pull secrets
Action item from https://gitlab.com/charts/gitlab/issues/473
Add a global settings for pull policy and pull secrets for GitLab charts.
At the same time we should update our default to not populate the pull policy. This will resort to using the k8s internal default of 'IfNotPresent' for our released images. And Always
for our master branch due to the use of the latest
tag. (description of default behaviour here: https://kubernetes.io/docs/concepts/configuration/overview/#container-images)