Rename local_build option to build_type for greater flexibility

[brienw]

When I implemented local_build in #229 I think I had the short-term blinders on. After giving it a bit more thought I think this should be implemented as a more general build_type config option that we can use into the future. Right now i see at least three use-cases:

build_type: :remote - (DEFAULT) build using remote build host
build_type: :local - run build locally
build_type: :docker - run locally inside a docker container (to be implemented in the future)