[openstack|network] Added missing Network model attributes

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.
Merge pull request #1581 from bvox/quantum-network-attributes

[openstack|network] Added missing Network model attributes