plugin-integrated test without nova

plugin integrated test without nova

Make quantum test of plugins run without nova (except vif driver?).
As there are plugins for software bridge (linux bridge/ovs) in the tree now
and more coming.
Since devstack/tempest require running nova right now, it takes long time,
it is fragile because of dependence of changeset of each tree.
So it has become difficult to test those plugins for software bridge.

So make them testable without nova, it will help the development as
- it will make quantum test faster
- it will make easier to test more advanced functionalities
  (live-migration, security group) with each plugins at lower level.
- it will make diagnose easier because it doesn't involves guest OS.

Necessary steps:
- replace nova with stub code
  probably an agent on each node.
  - creates/deletes ports (which maybe calls vif driver)
- make quantum existing test work without nova
  tempest? Is this worthwhile?
- add more tests
  e.g. live-migration without nova.
- hopefully tests can run on single machine by using name space to emulate
  multi host