Make warnings.warn test threadsafe
Make warnings.warn test threadsafe

warnings.catch_warnings is not threadsafe.

http://bugs.python.org/issue6647
https://docs.python.org/2/library/warnings.html#testing-warnings

Replace its use in our unit tests (which are gloriously multithreaded)
with a simple mock.patch.

Change-Id: I2864bdf5e0a9b29123e2261c4db03aba1e602566

pypowervm/pypowervm,powervm/pypowervm