Introduce security context related changes to nginx-ingress-controller to have nginx function without root user

Closes: #637 (closed)

Enabling running as non-root in images >= 0.16.0

• PR (still open) which does this to stable charts - https://github.com/helm/charts/pull/6745

Bumping image tag to 0.15.0

• PR which did this to stable charts - https://github.com/helm/charts/pull/5947
• 0.15.0 changelog: https://github.com/kubernetes/ingress-nginx/blob/master/Changelog.md#0150

For the stable chart, no change was made to bump to 0.15.0. Changelog doesn't mention any breaking changes.