SimpleConfigReader: ips are partitioned by link

We were already half-doing it, but this completes the process.

1) We don't maintain a list of ips or an ip->instance map
2) We add a new link,ip->instance map (link->ips list we had)
3) We add the link parameter to GetInstanceByIp (making it
   GetInstanceByLinkIp)
4) We change the GetInstanceByIp caller to pass None as link
   (thus for now using only the default link)

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>