Fix slave_hosts_cfg test
Fix slave_hosts_cfg test

It's failing due to new buildslaves running in our lab which aren't accounted
for in slave_hosts_cfg.py.

BUG=skia:2852
R=epoger@google.com, rmistry@google.com
TBR=epoger

Author: borenet@google.com

Review URL: https://codereview.chromium.org/472373005

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