Add a fullstack fake VM, basic connectivity test

Add a fullstack fake VM, basic connectivity test

* Full stack tests' fake VMs are represented via a namespace,
  MAC, IP address and default gateway. They're plugged to an OVS
  bridge via an OVS internal port. As opposed to the current
  fake machine class used in functional testing, this new fake
  machine also creates a Neutron port via the API and sets the
  IP and MAC according to it. It also sets additional attributes
  on the OVS port to allow the OVS agent to do its magic.
* The functional fake machine and the full stack fake machine
  should continue to share commonalities.
* The fullstack fake machine currently takes the IP address
  from the port and statically assigns it to the namespace
  device. Later when I'll add support for the DHCP agent
  in full stack testing this assignment will look for the dhcp
  attribute of the subnet and either assign the IP address
  via 'ip' or call a dhcp client.
* Added a basic L2 connectivity test between two such machines
  on the same Neutron network.
* OVSPortFixture now uses OVSInterfaceDriver to plug the port
  instead of replicate a lot of the code. I had to make a
  small change to _setup_arp_spoof_for_port since all OVS ports
  are now created with their external-ids set.

Change-Id: Ib985b7e742f58f1a6eb6fc598df3cbac31046951