Make it simpler to run Beaker tests

This makes it possible to run the acceptance tests for a particular OS via rake.

By default `:acceptance` runs the tests for Ubuntu, then CentOS. There are specific tasks for the individual OS types. I am forcing the Vagrant hypervisor to be virtualbox as that is what we currently have in our node config.

I should note that 'acceptance:centos64' currently fails as described in #26 and should probably be succeeding before #25 is merged.

I don't think we can run this on Travis as the only hypervisor that BEaker supports which has a chance of working on Travis is docker and that's not actually that easy, because you would need to install and configure docker within the travis container on each build.