[ADMIN_API]: Change update method on LBs for nodes

sqlalchemy did not like the way the updates were being
done to the loadbalancer table when nodes were down and
was throwning an exception. This method is cleaner and
sqlalchemy is not complaining.

Change-Id: I9ff4cb921b41c7fe5dc915f728368c873ff71a97
Merge "[ADMIN_API]: Change update method on LBs for nodes"