Apply recipes in multisite architecture

I added the --site-path option, which can be used to point the recipe to another site than the default.

Example:
php core/scripts/drupal recipe recipes/custom/basic_block --site-path subsite1
Assuming the path of the site is sites/subsite1.

Adding to the Phase 2 Roadmap. Thanks for your contribution!

I think this solution looks completely reasonable, but I think we could simplify the code a little bit. The --site-path option should have an internal default value of sites/default.