neutron_floating_ip to re-use FIPs

This patch makes neutron_floating_ip to try to re-use previously
allocated but disassociated floating IPs (FIPs) instead of always
allocating and associating a new one. A FIP will only be allocated
if there are no free (allocated, disassociated) ones.

Signed-off-by: Abel Navarro <abel@midokura.com>