Improve directory scaffolding; add .gitkeep files

[bradjones1]

Use a foreach loop, and also add a .gitkeep file to the directories.

Motivation - I tried to run tests using run-tests.sh and got some vague failures since Drupal's test script isn't particularly verbose. Tracked this down to core/tests/bootstrap.php which was trying to recursively search the profiles directory, which did not exist. That's because I had originally created this project on another machine, committed it to git, and then fired it back up on my current node. However, the empty directories created in post-install.sh did not get committed, per git's usual behavior.

The directories aren't re-created on composer install because web/autoload.php already existed. Empty directory, can't scan, makes PHPUnit unhappy.

[webflo]

Thanks, rebased the change in a0fd501.