ofagent: Switch to IptablesFirewallDriver

The latest version of ofagent
(ie. after blueprint ofagent-port-monitor)
follows IptablesFirewallDriver naming scheme.

Closes-Bug: #1364731
Related: blueprint ofagent-port-monitor
Change-Id: I690aab71b1cb222ffb8b458c90740ba623b0b37e
Merge "ofagent: Switch to IptablesFirewallDriver"