B #6299: Change SG flush timeout at VM poweroff

There is a race condition where, if a VM is powered off, its SG rules may not
be flushed in time. This may lead to the following type of errors

```
sudo -n ipset destroy one-VM_ID-NIC_ID-SG_NAME
```

The timeout to flush the security group iptables has been increased to 500 ms
to prevent this problem

src/vnm_mad/remotes/lib/security_groups_iptables.rb

@@ -616,7 +616,7 @@ module SGIPTables
         remove_chains_6.each {|c| commands.add :ip6tables, "-X #{c}" }
 
         # delay to allow kernel to clean up
-        commands.add 'sleep', '0.1'
+        commands.add 'sleep', '0.5'
 
         ipset_list.lines.each do |line|
             if line.match(/^#{chain}(-|$)/)