Reportstate 11

Provisionally fixes #11
Opening PR with the expectation that there is probably a bit more stuff to do, but some comments would be appreciated.
In particular, you have a test for `ReportStateScript.main` elsewhere, but not that it's hooked up to `flocker-reportstate` command line tool. Unfortunately testing that will be difficult (especially until #125 is fixed). So I guess don't bother with this for now and we'll rely on end to end integration tests.