Improved test coverage of haas.model.

Added tests which exercise the methods of everything the module except:

* the `Vlan` class, which needs to be ripped out in favor of `Network`,
* the `Port` class -- my intuition is that giving it its own label is
  excessive, and I want to revisit the interface to this.

In addtion, there is a config-related if-statement that is not covered,
in `init_db`.