Deprecate global.imagePullPolicy and move it to global.image.pullPolicy

What does this MR do?

This MR deprecates global.imagePullPolicy and replaces it with global.image.pullPolicy. It is part of a broad attempt to standardize global.image.* across different sub-charts.

Related issues

Part of #1617 (closed)

Test

The following values change every imagePullPolicy in the Chart. Use this for testing different scenarios:

helm template . -f build/test-ipp.yaml | grep imagePullPolicy

---
## build/test-ipp.yaml

certmanager-issuer:
  email: me@example.com

global:
  init:
    image:
      pullPolicy: pp-global-init
  image:
    pullPolicy: pp-global
  #imagePullPolicy: pp-global
gitlab:
  gitaly:
    image:
      pullPolicy: pp-gitaly
  gitlab-exporter:
    image:
      pullPolicy: pp-gitlab-exporter
  gitlab-pages:
    image:
      pullPolicy: pp-gitlab-pages
  gitlab-shell:
    image:
      pullPolicy: Never
  mailroom:
    image:
      pullPolicy: pp-mailroom
  migrations:
    image:
      pullPolicy: pp-migrations
  sidekiq:
    image:
      pullPolicy: pp-sidekiq
  task-runner:
    image:
      pullPolicy: pp-task-runner
  webservice:
    image:
      pullPolicy: pp-unicorn
registry:
  init:
    image:
      pullPolicy: pp-registry-init
  image:
    pullPolicy: pp-registry
minio:
  imagePullPolicy: pp-minio
  init:
    image:
      pullPolicy: pp-minio-init
nginx-ingress:
  controller:
    image:
      pullPolicy: pp-nginx-ingress-controller
  defaultBackend:
    image:
      pullPolicy: pp-nginx-ingress-defaultBackend

gitlab-runner:
  imagePullPolicy: pp-gitlab-runner
postgresql:
  image:
    pullPolicy: pp-postgresql
  metrics:
    image:
      pullPolicy: pp-postgresql-exporter
redis:
  image:
    pullPolicy: pp-redis
  metrics:
    image:
      pullPolicy: pp-redis-exporter
cert-manager:
  image:
    pullPolicy: pp-cert-manager
  cainjector:
    image:
      pullPolicy: pp-cert-manager-cainjector
  webhook:
    image:
      pullPolicy: pp-cert-manager-webhook
prometheus:
  alertmanager:
    image:
      pullPolicy: pp-prometheus-alertmanager
  configmapReload:
    image:
      pullPolicy: pp-prometheus-configmapReload
  kubeStateMetrics:
    image:
      pullPolicy: pp-prometheus-kubeStateMetrics
  nodeExporter:
    image:
      pullPolicy: pp-prometheus-nodeExporter
  server:
    image:
      pullPolicy: pp-prometheus-server

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened