Fixes #98: Add stable repo for grafana

[aidun]

Fixes #98: Add stable repo for grafana

Description

Add the addHelmRepo function to add stable ( https://kubernetes-charts.storage.googleapis.com ) Helm repository.

Motivation and Context

#98

• I have raised an issue to propose this change (required)

How Has This Been Tested?

helm repo remove stable
k3d create
./arkade install grafana
helm repo list

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

• I have tested this on arm, or have added code to prevent deployment