Make bucket names explicit in cq.cfg.

Don't assume bucket name to start with 'master.'.

NOTRY=True
BUG=chromium:617627
R=sergiyb@chromium.org

Review-Url: https://codereview.chromium.org/2096013002

infra/config/cq.cfg

@@ -17,7 +17,7 @@ verifiers {
 
   try_job {
     buckets {
-      name: "tryserver.infra"
+      name: "master.tryserver.infra"
       builders {
         name: "Depot Tools Presubmit"
       }