Add Vagrant test environment for cluster integration testing

This pull request adds a Vagrant environment to aid the testing of this role:

**Test with Ubuntu 12.04**

```bash
$ ANSIBLE_RIAK_OS="UBUNTU" vagrant up
```

**Test with CentOS 6.5**

```bash
$ ANSIBLE_RIAK_OS="CENTOS" vagrant up
```

After testing is complete, `vagrant destroy` gets rid of the Vagrant environment.

**Note**: There is optional support for the `vagrant-proxyconf` plugin, which can be used to cache assets pulled in via HTTP.