The following patch adds the following missing attributes:
+---------------------------+--------------------------------------+
| Field | Value |
+---------------------------+--------------------------------------+
| provider:network_type | gre |
| provider:physical_network | |
| provider:segmentation_id | 1 |
| router:external | False |
+---------------------------+--------------------------------------+
Not sure if the code style I've used for the attributes is OK though,
I did it so I don't break the 80 col. barrier.
Added also a small test for the new and existing attributes.
