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`.