1070585: GUI no longer locks on connection test. Adds cancel button.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1070585

The handlers for RestlibException and RemoteServerException could be combined:

```
except (connection.RemoteServerException,
             connection.RestlibException) as e:
```

Though that only works on python > 2.4, but...