User may associate fip to loadbalancer's vip_port when there is no
listener yet which means there is no virtual server yet on backend.
There is another case that user may create virtual server after
association of fip to vip_port. In this case, we need to use the
fip as the virtual server's vip ip_address.
This patch handles the above two cases which makes association of
floating ip to vip port gracefully.
Change-Id: Ic78d80109bbd0abf1b2b59786275cfa5b1a9d59c
073e7512b2