Checkmk Werk 11873: Fix exception caused by custom host attributes when saving host changes

ID: 11873 Title: Fix exception caused by custom host attributes when saving host changes Component: Setup Level: 1 Class: Bug fix Version: 2.1.0i1 When some kind of custom host attribute has been defined and is used in the host configuration, this may lead to an exception like this when editing or deleting hosts: <tt>Exception: AttributeError ('HostAttributeMYATTRIBUTEConcrete' object has no attribute 'is_explicit')</tt>.