Fix floating IP allocation for networks with a v4 *and* v6 subnet.

For networks that have both a v4 and a v6 subnet, the floating IP plugin
currently has two problems:

* When determining the subnet for the provided `internal_network_name`, it
assumes that the first item in the list of subnets is the one you want.
Instead, it should pick the first v4 subnet.

* When multiple fixed IP's exist for a given port (as is the case in a network
a v4 and a v6 subnet), neutron needs a hint as to which fixed IP to associate
to the floating IP address (the v4 one).