Replace tearDown with addCleanup in magnum unit tests

[Hua Wang]

Infra team has indicated that tearDown methods should be replaced with addCleanup in unit tests.
The reason is that all addCleanup methods will be executed even if one of them fails, while a failure in tearDown method can leave the rest of the tearDown un-executed, which can leave stale state laying around.

Moreover, tearDown methods won't run if an exception raises in setUp method, while addCleanup will run in such case.

So, we should replace tearDown with addCleanup methods.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/204925

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/204925
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=37af2d70f9f1c2a8454da20c5928bbbe8d1ed9a8
Submitter: Jenkins
Branch: master

commit 37af2d70f9f1c2a8454da20c5928bbbe8d1ed9a8
Author: Hua Wang <email address hidden>
Date: Thu Jul 16 03:03:43 2015 +0800

    Replace tearDown with addCleanup in magnum unit tests

    Infra team has indicated that tearDown methods should be replaced
    with addCleanup in unit tests. The reason is that all addCleanup
    methods will be executed even if one of them fails, while a failure
    in tearDown method can leave the rest of the tearDown un-executed,
    which can leave stale state laying around.

    Moreover, tearDown methods won't run if an exception raises in setUp
    method, while addCleanup will run in such case.

    So, we should replace tearDown with addCleanup methods.

    Change-Id: I3d16954ef7e2fd229f779d193f24ac01d1d2f641
    Closes-Bug: #1476976