remove superfluous by_type, by_group, by_arch methods from System

Change-Id: I89ad5882371997e76ecbc4615042786b0b450956
tests: refactored hostRequires tests to call needpropertyxml directly

These tests can bypass all the distro- and user-related filtering
because they only need to test the actual hostRequires filtering based
on system attributes.

This makes it easier to refactor the DistroTree.systems_filter method in
a subsequent patch.

Change-Id: I63ec252daa9ed35694afd18c2b719edc09acfffa
System query methods: permissable_systems -> visible_to_current_user

Change-Id: I5798f518bcdcc7a0304c85661022c2e631216fad
System query methods: DistroTree.all_systems -> System.compatible_with_distro_tree

Change-Id: Ib317650e997191ee3e572b0ad0a8bec7066f665c
System query methods: free -> is_free

Change-Id: Ide91ce1369ca5d90315c242e7cc056aa3ccdf27d
System query methods: available_for_schedule, _available -> available

Change-Id: Ia7f01ee0655ee54058fc9b2ab1a8c5a07171e7ab
System query methods: available -> can_reserve

Change-Id: If7f11a937dc1fd9c716e7eed0f99cc8d3f43fce7
System query methods: removed DistroTree.systems

Change-Id: I408e571206461a5a605327c10e1214580a97fa4d
System query methods: DistroTree.systems_filter -> Recipe.candidate_systems

Change-Id: Ib07e08ff9826f736e85dc1c5f04704f3dc8739f7