Design doc (internal, but progress on this bug will be external): https://docs.google.com/document/d/1NsLcO3W7YcOsI575xb4CTBtFmjiPo1xGRF_n-SqebuM/

Overview:
  * Add a `bundle` subcommand to recipe engine which allows a given set of repos (including any -O overrides) to be packed into a hermetic folder with a script that knows about the root package. This bundle can then be used to run any recipes that you'd be able to run with the regular ./recipes.py command, except that it does no network fetches.
  * Isolate the bundle
  * Teach buildbot about a "recipe_bundle" property which it will know how to call run_isolated on, with the arguments that it would normally pass to the recipe engine.
  * Build a local mechanism in depot_tools for folks to bundle+isolate their local recipes situation, and then trigger a bot with that and appropriate properties.