fleetctl should fail with a useful error when etcd is not exposing an accessible ip in redirects

When etcd is running on a NAT'd ip address, it's redirects will be unacceptable on the other side of the network address translation:

```
mac-mini:fleet jenkins$ curl -sL -i  "http://192.168.1.204:4001/v2/keys/_coreos.com/fleet/machines?recursive=true&consistent=true"  
HTTP/1.1 307 Temporary Redirect
Location: http://10.0.0.19:4001/v2/keys/_coreos.com/fleet/machines?recursive=true&consistent=true
Date: Tue, 06 May 2014 00:53:36 GMT
Content-Length: 127
Content-Type: text/html; charset=utf-8

``` 

In this case the image is running on OpenStack and did not have a floating ip address set at the time of instantiation.  This leads to a problem where etcd has no knowledge of the address it should be exposing to provide useful 300 level messages.