Add package provider for improved plugin handling #178

[TuningYourCode]

Pull Request (PR) description

This PR provides a package provider to manage grafana plugins as the current grafana_plugin resource does not offer all functions like installation of specific plugin versions or keeping plugins up to date with ensure => latest.

This functionality is now given by using

package { 'grafana-image-renderer':
  ensure => 'latest',
  provider => 'grafana',
}

or

package { 'grafana-image-renderer':
  ensure => '2.0.0',
  provider => 'grafana',
}

This Pull Request (PR) fixes the following issues

Fixes #178

Replaces #263

[TuningYourCode]

anybody experienced enough to write this test correctly? Already hat it like describe package('grafana-image-renderer') do but was also failing (probably because it doesn't set provider to grafana?)