Fix load balancer scenario if no tenant network

The load balancer scenario is currently dependent on isolated
tenant networking.  This updates it to allow it to pass in environments
where tenant networking is not present (ie, a shared cross-tenant network).
If a network cannot be resolved by tenant, it will be resolved by the
configured private network name as a fallback and its associated subnet
will be used.

Change-Id: Idbac7c10669a0e2db518223cdd762313aa752876
(cherry picked from commit 3e9e99fb7d7c43b5a6340919f4f1f611f387cbbb)
Merge "Fix load balancer scenario if no tenant network"