Align mocking pattern for test case

Mock sys.stderr/sys.stdout with mock.patch()

In novaclient/tests/test_shell.py, ShellTest.shell(), it is another
case that should be aligned. But it may induce test error if changed.
So, let's keep it as is.

Close-Bug #1237093

Change-Id: I8908dc1593dbbbaf2813cc8fb034be32972140a0
Merge "Align mocking pattern for test case"