Host route to metadata server with Bigswitch/Floodlight Plugin

Host route to metadata server was not getting propagated
to VM when using the Big Switch/Floodlight plugin. The
reason being the update to the subnet (with the host
route information) was not being picked up by the dhcp
agent. This is being fixed here by sending a notification
to the dhcp agent when the host route is added to the subnet.

Fixes: Bug #1134043

Change-Id: If040f51946bcb734da319249df17d03e68aa5588
Merge "Host route to metadata server with Bigswitch/Floodlight Plugin"