Made config set_val not mutate its argument
Made config set_val not mutate its argument

BUG=431173

Review URL: https://codereview.chromium.org/796493005

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293562 0039d316-1c4b-4281-b951-d872f2087c98

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