Added an `__init__` to haas.model.Vlan.

This fixes some failing tests.
Network/Vlan allocation mostly implemented at the model layer.

There are some failing api tests, which look like it's just that they
aren't making vlans first. I'm sure there are more tests that are worth
writing, but the code is roughly in the right shape now.
Stubbed out haas.api.vlan_register, and wrote some tests.
Implemented api.vlan_register
vlan_register test now expects a more specific error.
Got some tests passing.

We need to create vlans before networks now.