The current implementation of the ElectAuthServer doesn't handle well when the
default Registry server is not included in the response from the daemon Info
endpoint.
That leads to the storage and usage of the credentials for the default registry
(`https://index.docker.io/v1/`) under an empty string on the client config file.
Sample config file after a login via a Docker Daemon without Registry
information:
```json
{
"auths": {
"": {
"auth": "***"
}
}
}
```
That can lead to duplication of the password for the default registry and
authentication failures against the default registry if a pull/push is performed
without first authenticating via the misbehaving daemon.
Also, changes the output of the warning message from stdout to sdterr as
per dnephin suggestion.
Signed-off-by: Marcus Martins <marcus@docker.com>
Marcus Martins
Ignacio Capurro
Daniel Nephin