add --repo option to grafana_cli plugin install

[rwuest]

Pull Request (PR) description

Adding the repo option to the grafana_cli plugin installation, so that it's possible to use a different repository e.g. a company internal server to install grafana plugins. As an example:

grafana_plugin { 'grafana-simple-json-datasource':
  ensure    => present,
  repo => 'https://nexus.company.com/grafana/plugins',
}

This Pull Request (PR) fixes the following issues

n/a.

[bastelfreak]

Thanks for the PR @rwuest !