Extend validation relaxation introduced by commit 607afed94, by
allowing for multiple IPs for a given port also on DHCP and IPv6
subnets.
In order to ensure correct processing of DHCP bindings, the
process a binding is created only for the first IP address
allocated in the subnet; the process also ensures that the
address associated with the DHCP binding does not change unless
removed from the port's fixed_ips.
This change applies only to the nsx_p plugin, and the other
constraints on port IP allocation are still in place.
Change-Id: I0271b9be8e73e8e6b9d1b3b51bebc1542efd3d29
fe0e3089cb