load-balancer: binding refactorization

- remove the $*_internal parameters
- be able to specify multiple public VIPs in both $vip_public_ip and $vip_internal_ip
- allow the $*_api parameters to be either:
    true: listen on both public & internal VIPs
    false: don't listen at all
    one or more IP addresses: this service is bound specifically to these VIPs (check later that these VIPs are declared either in $vip_public_ip or $vip_internal_ip.)

Bug #237