Allow site setup to run on existing directories in www

Sometimes I have existing repositories that I want to setup in VVV. This pull request allows site setup on an existing directory, overwriting the configuration files in that directory.

It checks if the nginx config file for the site exists, and stops you there, if it does. You can delete that file to run site setup again, if needed. Also could add an option on the teardown process to only delete config files--but that seems overkill, for now.