Apply expand_root_placeholder to list, not per item.
Apply expand_root_placeholder to list, not per item.

TBR=agable@chromium.org,vadimsh@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/12851026

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@191084 0039d316-1c4b-4281-b951-d872f2087c98

luci/recipes-py,shishkander/recipes-py,luci/recipes-py,shishkander/recipes-py