client: create the output directory directory when ${ISOLATED_OUTDIR} is used
client: create the output directory directory when ${ISOLATED_OUTDIR} is used

Otherwise some test will be surprised by the fact that the output directory
doesn't exist and needs to be created. It is always created in advance in a
Swarming task.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org

Bug: 800899
Change-Id: Iff330cab2b1042845177e3789b3bc38a18f05166
Reviewed-on: https://chromium-review.googlesource.com/860628
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py