State change logging - fixes 296

Here's a partial fix / design which adds logging for errors in  `SetProxies` operations which would otherwise be consumed by the `DeferredList` and remain hidden from the user.

There are a couple of other examples of `DeferredList` s which consume errors and I guess these should be updated too:
 * flocker.cli.script.DeployScript._configure_ssh
 * flocker.cli.script.DeployScript._report_state_on_nodes
 * flocker.node._deploy.InParallel.run

Or perhaps I could create a more generic solution eg `LoggingDeferredList` ?