Fix str + list concatenation in trigger_swarm_step.py
Fix str + list concatenation in trigger_swarm_step.py

TBR=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11030054

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@160409 0039d316-1c4b-4281-b951-d872f2087c98

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