[openstack] Include auto-assigned IPs in floating_ip_addresses

In the OpenStack server model, the method floating_ip_addresses returns only manually-assigned public IP addresses, but omits IP's assigned via the auto_assign_floating_ip flag in nova.conf.  This patch allows floating_ip_addresses to return all floating addresses.

Repro A:
1. Configure auto-assignment of public IP's from a global pool
2. Launch an instance, and confirm the auto-assigned IP with 'nova list' (e.g. IP is 192.168.1.13)
3. Find server in fog interactive, then 'server.floating_ip_addresses'
Expect: ['192.168.1.13']
Actual: []

Repro B:
1. Configure auto-assignment of public IP's from a global pool
2. Launch an instance, and confirm the auto-assigned IP with 'nova list' (e.g. IP is 192.168.1.13)
3. Allocate a floating IP to the tenant (e.g. 192.168.1.17), and associate it with the new instance.  Instance now has two public IP's
4. Find server in fog interactive, then 'server.floating_ip_addresses'
Expect: ['192.168.1.17','192.168.1.13']
Actual: ['192.168.1.17']

Thanks for fog!