Update notes for submitted changes

* System query methods: DistroTree.systems_filter -> Recipe.candidate_systems
* System query methods: removed DistroTree.systems
* System query methods: available -> can_reserve
* System query methods: available_for_schedule, _available -> available
* System query methods: free -> is_free
* System query methods: DistroTree.all_systems -> System.compatible_with_distro_tree
* System query methods: permissable_systems -> visible_to_current_user
* tests: refactored hostRequires tests to call needpropertyxml directly
* remove superfluous by_type, by_group, by_arch methods from System