[openstack|network] create_network provider extensions

Implements provider extensions when creating networks.

See:

http://docs.openstack.org/trunk/openstack-network/admin/content/provider_attributes.html

Complements #1581 (Added missing Network model attributes)
[openstack|network] create_network provider extensions reworked

- Differenciate provider and vanilla options
- Fixed mocks
[openstack|network] remove extra trailing comma
Merge pull request #1598 from bvox/network-create-extensions

[openstack|network] create_network provider extensions