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.

(Review request: http://reviews.vapour.ws/r/4467/)

[davecheney]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7247

[jujubot]

Build failed: Does not match ['fixes-1568312']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7275

[davecheney]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7766

[davecheney]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7768

[davecheney]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7772

[davecheney]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Generating tarball failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7787

[davecheney]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/7789

[howbazaar]

Too much churn underneath to attempt to make good to land.