image.py, rootfs.py, package_manager.py: redirect stderr to stdout when calling check_output()
If a command executed with subprocess.check_output() fails, the
subprocess.CalledProcessError.output contains only STDOUT and the user
needs to check the log.do_rootfs to see any other details.
This commit forwards stderr to stdout so that, in case of failure, the
entire error output will be displayed in terminal.
