[Enhancement][opensearch-dashboards] Use global.dockerRegistry just like in the opensearch chart

[cpockrandt]

Is your feature request related to a problem? Please describe. I noticed that the opensearch and opensearch-dashboards chart use different approaches when it comes to the dockerRegistry. While I can set global.dockerRegistry in the opensearch chart, I need to overwrite image.repository in the opensearch-dashboards chart. Wouldn't it make sense to unify both charts, or is there a reason why the opensearch-dashboards chart doesn't use the global.dockerRegistry?

[rishabh6788]

@prudhvigodithi Please take a look.

[prudhvigodithi]

[Triage] Thanks @cpockrandt, similar to what we have here https://github.com/opensearch-project/helm-charts/blob/main/charts/opensearch/values.yaml#L27-L29, are you open to please contribute this? Thanks