Recipes: Use gclient checkout as a generator.
Recipes: Use gclient checkout as a generator.

BUG=

Review URL: https://codereview.chromium.org/180163011

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@254645 0039d316-1c4b-4281-b951-d872f2087c98

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