Replace tearDown with addCleanup in unit tests

Infra team has indicated that tearDown should not be used, and should
be replaced with addCleanup in all places. This patch removes the
few cases of tearDown in Ironic.

Change-Id: Ie2a59ea018c68fccadf0530409255bd9a0cf9717
Merge "Replace tearDown with addCleanup in unit tests"