all: prefer AddCleanup to TearDown

[davecheney]

A common failure mode of our tests is when an issue occurs during the
SetUp phase of the suite or test, not only does the test fail, but the
Tear down phase blows up because it is rarely written to handle the case
where the setup phase did not complete.

This PR is a toe in the water to experiment with applying a philosophy
of moving work from the TearDown phase of the test suite to a
'defer-like' AddCleanup style.

(Review request: http://reviews.vapour.ws/r/4465/)

[davecheney]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju