Changes to tox in the past year have rendered the PBR-specific
tweaks in get_sdist_name invalid, since calling tox from within tox
now creates testenv directories relative to the parent tox's working
directory rather than the child's. While we're here, turn on more
verbose logging in the tox invocation, and fix the subprocess
routine to not discard the command's output.
Change-Id: I35fb5dde9ec400397ed21035d57c3efb872d8e58