Locking nginx site configuration files in Homestead.yaml

Allow locking a site in Homestead.yaml so it's existing nginx site configuration file (/etc/nginx/sites-available/example) wont be overwritten on the next vagrant provision. (Useful for non or more custom Laravel projects powered by Homestead)

### Example
```yaml
# Homestead.yaml

sites:
    - map: example.dev
      to: /home/vagrant/web/example
      locked: true
```