Move provision_drupal_system_map() to the post verify drush hook, and only run if drush option "installed" is true.

 - Adding chdir before _provision_drupal_create_directories() when in site context, otherwise cwd is the sites folder.
 - Make sure to only act on sites.
 - Removing drush_bootstrap(DRUSH_BOOTSTRAP_DRUPAL_SITE) and _provision_drupal_ensure_htaccess_update() because they both require the site to be installed.

Refactoring _provision_drupal_site_exists() to check for Drupal Bootstrap instead if a settings.php file exists.

Remove drush_provision_drupal_pre_provision_install_rollback() and replace with drush_provision_drupal_pre_provision_verify_rollback().

drush_provision_drupal_pre_provision_verify_rollback() must check for a bootstrapped site, or a single verify fail would destroy the site's config.