Reduce security group db calls to neutron server

Within ovs agent daemon loop, prepare_devices_filter will impose heavy workloads
to neutron server in order to retrieve the security groups message to apply
firewall rules. If agent is configured to use Noopfirewall driver or security
groups are disabled, there is no need for loading the rules from server and
refreshing the firewalls. This will reduce the number of db calls and improve
performance for neutron server in this case.

Change-Id: Id244aab3cac37fc6ed3dc05cbee91cdf9e34d9cc
Closes-Bug: #1365806
Merge "Reduce security group db calls to neutron server"