[LP#1905008] Use subnet cidr for member secgroup rules to support amphora VRRP_IP

[addyess]

This fixes LP#1905008 by using the subnet cidr for member rules allowing the health checks to succeed since they use the amphora VRRP_IP and not the ip of the loadbalancer.

[rapour]

Thanks a lot @addyess, my assumption is that both LB VIP and amphora VRRP IP reside in the same CIDR and by extending the security group rule Neutron will pass the traffic to kube api, which LGTM. Just left a question out of curiosity