When a fully populated LB is created, if the member is not created
indicating the subnet_id to whom it belongs, the LB vip_network_id
is inherit by error as member.subnet_id.
This patch fix this behaviour to inherit the member.subnet_id from
the loadbalancer.vip_subnet_id that is always passed from Octavia
API when call is redirect to the OVN-provider.
Closes-Bug: #2079813
Change-Id: I098afab053119d1a6eac86a12c1a20cc312b06ef