add variable datasource to fix error

[RuStyC0der]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Hello
I have tried to provision your dashborad to grafana (https://grafana.com/docs/grafana/latest/administration/provisioning/#dashboards) and got an error:
"Error updating options: Datasource ${DS_PROMETHEUS} was not found"
I have add variable datasource to fix this issue. This variable datasource will be also usfable when you have multiple prometheuses connected to your hrafana.

[RuStyC0der]

I have also update dashboard`s screenshot according to add datasource field. Since I'm using a laptop in the screenshot, you can't see the fan speed and power draw. You may need to update the screenshot on your computer

[utkuozdemir]

Thanks for the PR. I'll have a look when I have some time.