VM cannot ping internet

I know this question was asked several times. Therefore I spent an hour reading previous posts. None seem to describe my case. The closest seems to be [1] but I don't understand why they have both br-eth0 and br-ex. Shouldn't it work fine if we rename the original br-eth0 to br-ex? The manual for the minimal 3 node neutron installation seems to say everything should work with br-ex only [2]

I did not want to follow up the above original question as it was asked more than a year ago. So, let me explain my case.

I am running Juno minimal neutron installation with 3 nodes (controller, network, compute1); just as in the manual [3]. Nodes are running Ubuntu 14.04 as a VirtualBox VMs on my Desktop. I successfully created OpenStack VMs and they can ping, ssh each other though local network (on 10.0.10.0/24 subnet). But OpenStack VMs can not access the external network.

My network node's eth0 is connected to internet via VirtualBox NAT (on 10.0.2.0/24 subnet), which is used as the port of the br-ex (see content of network node's /etc/network/interface here http://ibin.co/2275uzBhiJOW). Note that I manually made br-ex entry on the file because for some reason br-ex did not automatically get IP address when configured via OVS. I can access the Internet from network node via br-ex on 10.0.2.15.

ovs-vsctl shows br-ex as correctly bridged on eth0 (see the screenshot http://ibin.co/225brKO2Rfq0). Here is my OpenStack virtual network topology http://ibin.co/227AzhZCNIgQ. OpenStack VMs can ping router's external address on 10.0.2.50 but cannot ping any public DNS such as 8.8.8.8.

First, I checked if packets are reaching br-ex. Apparently no, when I ping 10.0.2.15 (br-ex IP address) from the OpenStack VM and do tcpdump -i any -n host 10.0.2.15 on the network node, it shows ARP request messages on br-int (see the screenshot http://ibin.co/227BqSzHSFqI) but nothing shows up on tcpdump -i br-ex -n host 10.0.2.15

Network node's ip route outputs:
default via 10.0.2.2 dev br-ex
10.0.0.0/24 dev eth1 proto kernel scope link src 10.0.0.21
10.0.1.0/24 dev eth2 proto kernel scope link src 10.0.1.21
10.0.2.0/24 dev br-ex proto kernel scope link src 10.0.2.15

Can someone please point what is going wrong? Any help is appreciated.

Nodir

[1] [ovs] br-ex confusion and neutron network https://ask.openstack.org/en/question/24853/ovs-br-ex-confusion-and-neutron-network/
[2] Juno neutron basic installation](http://docs.openstack.org/juno/install-guide/install/apt/content/ch_basic_environment.html#basics-networking.
[3] OpenStack Juno networking with Neutronhttp://docs.openstack.org/juno/install-guide/install/apt/content/ch_overview.html