SimpleConfigReader: add IP address lookup helpers

Add the following functions to SimpleConfigReader:
GetInstanceByIp, nic ip -> instance name
GetNodePrimaryIp, node name -> node primary ip
GetInstancePrimaryNode, instance name -> primary node

For the first one we precalculate the _ip_to_instance, so we don't have
to loop over all instances for each query.

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