fix ansible-playbook runs when ansible.verbose is not specified

The way that the ansible.verbose option was implemented, it was passing an empty set of quotes to the ansible-playbook binary, which caused it to be unable to find the specified playbook.

Given a Vagrantfile containing:
```
  config.vm.provision "ansible" do |ansible|
    ansible.host_key_checking = "false"
    ansible.playbook = "playbook.yml"
  end
```

The following command was generated:
```
 INFO subprocess: Starting process: ["/usr/local/bin/ansible-playbook", "--private-key=/Users/jdoe/.vagrant.d/insecure_private_key", "--user=vagrant", "--inventory-file=/Users/jdoe/git/ansible-saf-playbooks/vagrant/safeassign/vagrant_ansible_inventory_default", "", "playbook.yml"]
```

Which resulted in the following ansible-playbook error:
```
[default] Running provisioner: ansible...
ERROR: the playbook:  could not be found
Ansible failed to complete successfully. Any error output should be
visible above. Please fix these errors and try again.
```

This patch fixes the ansible.verbose parameter so that it is optional and defaults to not being set.  The default logging level of Ansible is good as is and the verbose parameter can be used to increase it as needed.