Mock time.sleep for the IPMI tests

The commit 53640a873000a12eebd627df555e7b2b0a27f659 introduced a interval
between IPMI commands to make sure that the BMC doesn't get bombarded
with requests, but not all tests are mocking the sleep command so now it
takes a lot more time to run. This patch is fixing this time problem by
mocking time.sleep at the class level.

Change-Id: I8810df1b59a0f650f68cc77b65667d59f1f6dd1e
Closes-Bug: #1332000
Merge "Mock time.sleep for the IPMI tests"