Checkmk Werk 12301: Fix cache usage for clusters

ID: 12301 Title: Fix cache usage for clusters Component: Checks & agents Level: 1 Class: Bug fix Version: 2.1.0i1 The cache was not used properly for clusters. This resulted in performance problems such that, if an host was part of <i>n</i> clusters, it would be contacted <i>n + 1</i> times. Now, we make sure that the host is contacted a single time and that checking for the clusters use locally cached data.