Automatic merge from submit-queue

cloudprovider/openstack bug fix: don't try to append pool id if pool doesn't exist

**What this PR does / why we need it**:

This fixes a bug in the OpenStack cloud provider that could cause a panic.

Consider what will happen in the current `LbaasV2.EnsureLoadBalancerDeleted` code if `nil, ErrNotFound` is returned by `getPoolByListenerID`.