Interactive `bst init` doesn't create user provided `element-path` folder

Summary

Spotted this while trying to make use of bst init in some kind integration testing with @chiaratolentino

It asks for the element-path. It uses it in the generated project.conf, but creates the folder with the default name elements and we actually end up with a wrong configuration.

Steps to reproduce

$ bst init
# enter a project name
# enter version
# enter an element-path different than `elements`, like `custom`.

$ cat project.conf
.. # contents of project.conf
element-path: custom
..

$ ls -l
elements project.conf # ??

What is the current bug behavior?

See above

What is the expected correct behavior?

With the same reproduce steps above,

$ cat project.conf
.. # contents of project.conf
element-path: custom
..

$ ls -l
custom project.conf

Possible fixes

This seems like caused by the very similar naming of two variables (elements_path, element_path) here in app.init_project()

I think easiest fix would be moving

elements_path = os.path.join(directory, element_path)

close to the creation of that dir, for example:

            elements_path = os.path.join(directory, element_path)
            try:
                os.makedirs(elements_path, exist_ok=True)
            except IOError as e:
                raise AppError("Error creating elements sub-directory {}: {}"
                               .format(elements_path, e)) from e

Other relevant information

• BuildStream version affected: 1.3.0+703.g892866e1

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