…a dashboard spec

There's a fallback logic in (r *DashboardPipelineImpl) obtainJson() that allows a user to have multiple sources for configuration with falling back to the next one in case of an error. For unknown reasons, having both url and grafanacom in spec was not allowed. This restriction should be lifted.

…ixes

- for code simplification, moved from &metav1.Duration to metav1.Duration where the former was not really needed (GrafanaDashboardStatus, GrafanaSpec);
- fix: for unknown reasons, NewDashboardPipeline would fallback to 24h instead of 0(=infinity) if dashboard.Spec.ContentCacheDuration is not defined. Removed that;
- moved cache extraction from obtainJson() to loadDashboardFromURL(). - I think that location makes more sense;
- fix: don’t use dashboard cache if url is different.