Fix floating ip migration issues

Fix floating_ips dict indexing
Add retry mechanism for adding floating ip to moved server (required
due to https://bugs.launchpad.net/nova/+bug/1249065)
Fix logic of migrate_floating_ip to return correct floating ip obj

As nova.floating_ips_bulk.create() method returns an ip range dict,
we need to duplicate find() request after we add a floating ip so
we could return actual instance of FloatingIP class.
Add 1s delay between attempts to add floating ip to server