Allow running bst commands from project sub-directories

This is similar to #341 but the difference here is that instead of running commands from the workspace directory, I would like to run bst commands from sub-directories of the project. At present, users are forced to run any bst commands from the root of the project. This means that if you are editing the config for an element which happens to live in a sub-directory, you have to switch back to the project root to test those changes.

It would be nice if BuildStream could try to find project.conf in parent directories unless it hits /.