Service filtering is once again working.

- Reordered filtering to consider units first. This drives all other filtering.
- Included service filtering tests once more.
- Change in behavior: machines which are not filtered will now list all their subordinates.
Merge branch 'master' into 1385456-services-no-longer-filtered
Implemented peer-review suggestions.

- Obviated need for bounds checking by building up a slice instead of tearing one down.
- Behavior of service filtering as it was before new filtering infrastructure.
Merge pull request #1027 from katco-/1385456-services-no-longer-filtered

Service filtering is once again working.

- Reordered filtering to consider units first. This drives all other filtering.
- Included service filtering tests once more.
- Change in behavior: machines which are not filtered will now list all their subordinates.
- https://bugs.launchpad.net/juju-core/+bug/1385456