[buildbucket] specify swarming hostname

it is required now

TBR=dpranke@chromium.org

Bug:
No-Try: true
No-Presubmit: true
Change-Id: I66b9796cc0cc5abfe4e93f9a5aa5e8101e3478b1
Reviewed-on: https://chromium-review.googlesource.com/812105
Reviewed-by: Dirk Pranke <dpranke@chromium.org>