Get pool members from DB API

When the Load Balancer members are created as a result of autoscaling
groups calling _lb_update, the reference to the current members is
stored in the database and not in the template.  Treat ResourceData
mappings as canonical.

Closes-Bug: #1248218
Change-Id: Iaf8097eff6ad5ae0809d56604699dca2b08db3f4