[FLOC-1070] Acceptance test runner.

The tutorial Vagrantfile hard-codes the version of the tutorial base box, which means that it needs to be edited to run tests against other versions.

This provides a new Vagrantfile, and a test runner that invokes it. Hopefully, the new runner should also be able to deal with cloud VMs eventually too.
This is a useful feature for running specific tests and it'd be nice to add this to the `--help` synopsis.
```
(1070)(acceptance-runner-FLOC-1070 =)[~/.../HybridLogic/flocker]$ ./admin/run-acceptance-tests --help
Usage: run-acceptance-tests [options]
Options:
  -k, --keep           Keep VMs around, if the tests fail.
      --distribution=  The target distribution. One of fedora-20.
      --provider=      The target provider to test against. One of vagrant.
                       [default: vagrant]
      --version        Display Twisted version and exit.
      --help           Display this help and exit.

Run the acceptance tests.

```