Used checked in configs with rbe_autoconf when possible (#276)


For users of the rbe ubuntu 16_04 containers, attempt to first find if a published config will work for their chosen Bazel version. If such a config exists, rule only creates BUILD files with aliases pointing to the published configs.

Pulling container / generating configs will only happen if the above step does not find a published config.

Behavior can be overriden with use_checked_in_confs = False,