Shift db_import to run after wp install

**The problem.** The command 'wp core is-installed' seems to interpret the presence of an imported db to mean WP is already installed, so 'wp core install' doesn't end up running and the site won't load.

**Potential solution.** This commit moves the 'Import database' play to a handler to be run at the end of the 'wordpress-sites' role (so the db imports _after_ 'wp core install'). The handler is "notified" to run via the 'Copy database dump' play in roles/wordpress-sites/tasks/database.yml.

**Note.** For better or worse, a 'vagrant provision' command will not re-import or overwrite a database that has already been imported.