Add support for crontabs provisioning and custom nginx-server settings through Homestead.yaml.

Make it possible to define crontabs and getting them installed for 'root' on VM: (minute/hour/monthday/week/weekday defaults to '*' if left out)
```
crontabs:
    - name: import-addresses
      command: /home/vagrant/Code/jobs/addresses-import-job.sh
      minute: '*/15'
      hour: '*'
      monthday: '*'
      week: '*'
      weekday: '*'
```

And extend nginx settings like this:
```
sites:
    - map: homestead.app
      to: /home/vagrant/Code/Laravel/public
      client_max_body_size: 100M
```