Unit test for the VirtualTopology

Adds a unit test for the agent virtual topology, currently
testing only the ports observable.

To support the new storage model, the unit test introduces
the in-memory mock implementation of the ZOOM Storage trait
in the MidolmanSpec test base class.

Change-Id: I86b916072efdb4fe4091e04e8f96d651773033b8
Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>
Merge "Unit test for the VirtualTopology"