Change WP-CLI commands so they run as the www-data user

Got the following error when running initial VU command. WP core install command runs from the wrong directory.

SSH'd in, CD to /vagrant and ran the command manually - WP installed all good.

```
err: /Stage[main]//Sennza::Wp[vagrant.local]/Sennza::Site[vagrant.local]/Wp::Site[/vagrant/wp]/Exec[wp install /vagrant/wp]/returns: change from notrun to 0 failed: /usr/bin/wp core install --url='http://vagrant.local/' --title='WordPress Site' --admin_email='admin@example.com' --admin_name='admin' --admin_password='password' returned 1 instead of one of [0] at /tmp/vagrant-puppet-1/modules-0/wp/manifests/site.pp:29
```