[Test-Support] Add Vagrant configuration for testing in VM

This PR adds a Centos 6 Vagrant box that can be used for easy integration testing when working on garethr-riemann.

There is some overlap to riemann-vagrant but having an integrated Vagrant configuration simplifies integration testing a lot.

I have not written any tests because I couldn't find a sensible thing to test. If you have something you'd like to have tested, I'll gladly provide that.