Add -host-ip argument for cases when the hostname resolves to docker0

The containerized version of registrator sets the IP address of the service as the hostname, which resolves to the docker0 address, and not the externally visible address. Added -host-ip to allow that to be fixed. 

Also added a log message to indicate the backend + uri, since I kept setting it wrong and it would hang because the uri was wrong. :) 
I actually think it should be a global reference.