Coveralls 119

See 54.191.139.87 for this in action, and the badge at https://coveralls.io/repos/ClusterHQ/flocker/badge.png.

The issue with this is that Buildbot builds a coverage report for a given branch merged with master. This means that the coveralls link on a branch's build's coveralls page is a link to a merge of master which isn't pushed to the PR so it is a 404 link.

As well as what can be seen here see the https://github.com/ClusterHQ/flocker/compare/coveralls-119 branch for `flocker`, and the FlockerBuildbot Credentials in LastPass.

Thanks @tomprince for help.

I will deploy this when it has passed review.
If `%(prop:workdir)s` isn't filled then a lot of other features won't work, so I'd like to keep it.