Fix slave_machine.py locating its dimensions file by setting cwd.
Fix slave_machine.py locating its dimensions file by setting cwd.

Make the current working directory deterministic to the directory containing the
zip file, this simplifies assumptions.

R=csharp@chromium.org
BUG=swarming:109

Review URL: https://codereview.appspot.com/104750044

pombreda/swarming,madecoste/swarming,madecoste/swarming,luci/luci-py,pombreda/swarming,luci/luci-py,pombreda/swarming,luci/luci-py,madecoste/swarming,luci/luci-py,madecoste/swarming,pombreda/swarming