Support overriding the --tls-min-version controller manager flag in the Helm chart

[cbuto]

Describe the solution you'd like
The --tls-min-version flag was added in #1424; however, there is not an easy way to set this flag using the gatekeeper helm chart.

I'd like to be able to pass in a value to the Helm chart to set the flag to --tls-min-version=1.2.

Anything else you would like to add:
Instead of adding the option directly in the args, should the chart support passing any arbitrary arguments (extra args) to the deployment?

i.e.

{{- if .Values.extraArgs }}
{{ toYaml .Values.extraArgs | indent 8 }}
{{- end }}

Environment:

• Gatekeeper version: 3.8.1
• Kubernetes version: (use kubectl version):