launch_slaves: fix new-style buildbot gclient config
launch_slaves: fix new-style buildbot gclient config

BUG=skia:2999

Review URL: https://codereview.chromium.org/633363003

Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot