Support multiple service frontends

[SpComb]

A service could have multiple frontends, configured as /clusterf/services/foo/frontends/bar. This would support the case of having multiple frontend load-balancers, each with their own address, forwarding traffic to the same set of backends.

clusterf-ipvs probably needs some option to select the subset of frontends configured on the machine, to omit IPVS services for frontend addresses that are only configured on some subset of the machines.

[SpComb]

Mind you, this would quickly lead to an O(F * B * k) explosion of IPVS destinations over the number of frontends x backends x {ipv4,ipv6}:{tcp,udp} pairs.

An alternative might be the use of IPVS fwmark services, but that would require iptables integration, which becomes far more complicated.