two pass build

[robnagler]

Components need to communicate. Right now this is an ad hoc collection of calls, e.g. docker_cache.update_j2_ctx, nginx.install_vhost, and nginx.update_j2_ctx_and_install_access. Splitting the build into compile and write would allow simpler and clearer communication (including better assertions e.g. you can't have docker_cache and docker_registry on same host).

build.create_host should call require_component then write_component. components call back to the build object to add themselves to the write queue. Initially, this would be backward compatible, but eventually, two-pass should be required. Components currently have a compile and then write implementation, which wouldn't need to be split initially.