Support multiple host names

[baalexander]

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.

[steverice]

I've been using this pull request without issue. The syntax is exactly what I'd expect and, being new to Vagrant and Ansible, was surprised it's not built in.