CustomVarRendererHook implementation must also be able to blacklist properties

[raviks789]

Currently, the monitoring module renders all the the custom variables in an object. This in turn also renders the variable _override_servicevars as shown below, which was introduced by Icinga Director module to conveniently override the custom properties of services in a host. And hence it should not be displayed in the UI. Moreover, as shown in the screenshot it exposes the values of custom properties whose visibility is actually hidden.

This is currently solved by blacklisting the property _override_servicevars in the monitoring module.

But this should be instead handled in the CustomVarRendererHook implementation by adding a feature to blacklist custom variables.