Add load balancer PoolMember resource

This adds a new resource to manage the relationship between a load
balancing pool and an address, allowing more options like setting the
weight of the member.

Change-Id: Ic8a55ff8b8b00c7a4e3184b3028d5bf0cdd5c916
Implements: blueprint loadbalancer-pool-members
Merge "Add load balancer PoolMember resource"