Default clone_repo to master if branch not found

Behavior change from set -e causes attempt to checkout an invalid
branch to fail. Explicitly fall back to master if the requested
branch name checkout fails.

Change-Id: Ib20a4910275126fc3949b06dbc91a301c2d04406

jenkins/scripts/release-tools/clone_repo.sh

@@ -169,6 +169,6 @@ else
     # directory already existed we might have checked out something else
     # before so just do it again).
     (cd $local_dir &&
-            git checkout $BRANCH &&
+            (git checkout $BRANCH || git checkout master) &&
             git pull --ff-only)
 fi