fix/notification-command-inherited-fields-missing refs #2286

[moreamazingnick]

Icinga director has different relations for Checks and Notifications but references the same db-table
for checks: check_command
for notifications: command

IcingaObjectfield loader only checks relation check_command

[moreamazingnick]

seems to be more complicated since the var resolver doesnt involve inheritance from a notification command. but the inheritance works

[moreamazingnick]

If we patch protected function eventuallyGetResolvedCommandVar(IcingaObject $object, $varName) ind DIrectorDatafield the resolver works works fine.

@Thomas-Gelf Please tell me if I should prepare another merge request, or if this inheritance behaviour is not a bug but a feature