update helm charts for helm-dashboard

[warjiang]

1. fix imagePullSecrets bug in deployment.yaml
2. add global config
3. optimize image field with global registry config

Changes Proposed

this pr is for charts of helm-dashboard

1. fix imagePullSecrets bug in deployment.yaml
   
   if use old deployment.yaml it will reported that e expect map but get string

2. add global config
   busybox image is hard code in the charts, so i propose to use global.imageRegistry to unify render of image. same with imagePullSecrets

Check List

• The title of my pull request is a short description of the changes
• This PR relates to some issue:
• I have documented the changes made (if applicable)
• I have covered the changes with unit tests

[undera]

Thank you!