Checkmk Werk 11117: Drop support for creation of HostLabels via discovery function

ID: 11117 Title: Drop support for creation of HostLabels via discovery function Component: Checks & agents Level: 1 Class: New feature Version: 2.0.0i1 This werk only affect users who implemented their own plugins *and* where creating HostLabels using the discovery function. Plugins trying to create HostLabel instances will fail with a NameError. The afore mentioned feature is no longer supported. Creation of host labels can now only be implemented using the new check API. It is done by passing a `host_label_function` to the `register.agent_section` function. For details please refer to the dokumentation.