Do not lookup l3-agent for floating IP if host=None, dvr issue

If a floating IP has been associated with a port, but the port
has not been associated with an instance, attempting to lookup
the l3-agent hosting it will cause an AgentNotFoundByTypeHost
exception.  Just skip it and go onto the next one.

Change-Id: If3df9770fa9e2d2eada932ee5f243d3458bf7261
Closes-Bug: #1370795
Merge "Do not lookup l3-agent for floating IP if host=None, dvr issue"