Comment 5 for bug 1525059

Our team did some investigation. In our change, it's quite simple and will not need to change L3 agent's gateway business logic.

How to do it?
1. Set "external_network_bridge" to empty.
   Router gw port "qg-*" will connect to br-int.
   If not, "qg-*" will connect to external_network_bridge(for example: br-ex), it will be complex.
   For example, 2 "qg-*" in 2 different vxlan networks need vlan isolation, but br-ex did not have yet.

2. L2 agent will detect the qg-* port, it will found it's gw port and gw network is vxlan network.
   It will create tunnel in br-tun, and install the flow which can reuse current APIs.

We almost finish the test on ovs agent, if necessary, we can share the code and write a function spec.