Add merge_stderr option to all client methods

This commit makes the client methods behave more uniformly. Before
there only certain methods had a kwarg for merge_stderr, which could
lead to unexpected behavior depending on the clients being used. By
adding the option everywhere there shouldn't be an unexpected behavior
around merge_stderr.

Change-Id: I227d5e684c94da29a47eb9876a9ed40cdbea084c