Use global rebar binary with fallback to local rebar binary

[jhg]

I'm developing a Docker base image with Erlang and Rebar installed and shared between all images that use it as base. The idea it's copy only the minimal files for the container to be small, then if Chicago Boss can use the global Rebar binary the Dockerfile don't need copy the rebar binary again.

Also it's useful for example for developers that use a rebar binary globally. I have it and really I don't need versioned rebar binary, it's a best practices not version binaries and third libraries or tools in git repository.

This changes allow to use a global binary of Rebar but still it can run local Rebar binary if not found the global binary.