Support multiple host names

Some of my playbooks belong to different ansible hosts and I would like to install them all using vagrant-ansible.

This pull request lets you specify `ansible.hosts = ["host1", "host2"]` in the Vagrantfile, which generates an inventory file looking something like:

```
[host1]
localhost:22

[host2]
locahost:22

```

Single host syntax (`ansible.hosts = "host1"`) is still supported.

I rarely write Ruby code, so please let me know if you need anything changed.