cq.cfg: enable experimental swarming builders

infra/config/cq.cfg

 # See http://luci-config.appspot.com/schemas/projects/refs:cq.cfg for the
 # documentation of this file format.
 
 version: 1
 cq_name: "luci-py"
 git_repo_url: "https://github.com/luci/luci-py.git"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 rietveld {
   url: "https://codereview.chromium.org"
 }
 verifiers {
   reviewer_lgtm {
      committer_list: "project-infra-committers"
   }
   try_job {
     buckets {
       name: "master.tryserver.infra"
       builders { name: "Luci-py Presubmit" }
+      builders {
+        name: "Luci-py Presubmit (Swarming)"
+        experiment_percentage: 100
+      }
     }
   }
 }