Cleanup gearman clients in tests

Zuul RPCClient tests used an in test gear client to interact with the
gearman server fixture and make Zuul RPC requests. These clients weren't
explicitly shut down so their threads were leaking. Fix this by adding
cleanups to these tests that shutdown the clients after the test has
run.

Change-Id: Ib936f766fd1b87734cc2f925f462437b95098bc5
1 file changed