Force cmd to be a tuple of strings.
Force cmd to be a tuple of strings.

We handled cases where cmd was not a tuple or not all strings, we just didn't
handle them early enough. Now we do.

R=xusydoc@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15720008

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@202213 0039d316-1c4b-4281-b951-d872f2087c98

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