Better output on SSH configuration failures - fixes 424

Fixes #424.

Here's what output looks like now:

```
$ flocker-deploy minimal-deployment.yml minimal-application.yml 
Error connecting to cluster node: ssh: connect to host 172.16.255.250 port 22: No route to host
```
Ah. So that's all you need to get the stderr included in the `CalledProcessError` ? And you're using `check_output` to prevent the stdout being printed under normal circumstances. Maybe a comment would be in order to explain why we go to the trouble of capturing stderr and then *apparently* discard it.