Add a default beaker node

When running `bundle exec rake beaker` without the `BEAKER_set`
environment variable, you encounter a horrible error:

```
$ bundle exec rake beaker
/opt/rubies/1.9.3-p545/bin/ruby -S rspec spec/acceptance --color
/opt/rubies/1.9.3-p545/lib/ruby/gems/1.9.1/gems/beaker-1.17.2/lib/beaker/options/hosts_file_parser.rb:23:in
`parse_hosts_file': Host file
'/Users/ssharpe/Projects/gds/puppet-graphite/spec/acceptance/nodesets/default.yml'
does not exist! (ArgumentError)
```

You do not encounter this error when running `BEAKER_set=BEAKER_set=centos-64-x64 bundle exec rake beaker`

The default nodes to test on is our ubuntu1204 definition.

FWIW, while this might get us closer to running on Travis, I'm still not
sure that's possible because I don't think Travis supports any of the
hypervisors in [hypervisor.rb](https://github.com/puppetlabs/beaker/blob/master/lib/beaker/hypervisor.rb)
Of course I am happy to be proven wrong, as long as it's in the form of
a pull request ;o)
Make it easier to run Beaker tests

This defines some new rake tasks:
- 'acceptance': runs the tests for all OS types
- 'acceptance:ubuntu1204': runs the tests for Ubuntu
- 'acceptance:centos64': runs the tests for CentOS
Merge pull request #28 from gds-operations/beaker-default-nodes

Make it simpler to run Beaker tests