Add nodes IPs informations to ssconf

Having a list of primary/secondary IPs of all the nodes in ssconf can be useful
for scripts/hooks which need to automatically configure network properties for
the whole cluster (e.g.: ipsec/netfilter rules) without relying on a
working DNS.

Signed-off-by: Luca Bigliardi <shammash@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>