OVS agent: flush firewall rules for all deleted ports at once

OVS agent: flush firewall rules for all deleted ports at once

In some cases, under high load OVS agent has to delete a big amount of
ports during rpc_loop. remove_devices_filter() does iptables-save/restore
for IPv4 and IPv6 which is 4 system calls. It is very expensive and
inefficient to call it for each port individually.

Closes-Bug: #1491922
Change-Id: I4cfb2dfcef5088436c7aaae22c8f66e1ea052311