check: add custom cluster name

Custom cluster names are not supported anymore by the Ceph project. Warn (or should we consider make it an error?) when a custom cluster name is found

For reference: https://bugzilla.redhat.com/show_bug.cgi?id=1459861

Start by locating the existing cluster-name check in the ceph-medic codebase and review how checks report warnings and errors. Read the linked Bugzilla reference and determine whether a custom name should warn or fail; done means custom cluster names are detected with the agreed behavior.

Written by the indexing model from the issue text.