Update cloud networks docs with attach/detach vif examples

It is now possible to attach/detach cloud networks on existing Rackspace cloud servers. See here:
http://docs.rackspace.com/networks/api/v2/cn-devguide/content/api_create_virtual_interface.html

This is implemented as a nova extension that's automatically installed by rackspace-novaclient, so this just updates the docs accordingly. Calling 'pyrax.cloudservers.os_virtual_interfacesv2_python_novaclient_ext' is kind of ugly, but it works. There may be a prettier way to do the same thing that I'm unaware of.

The limits have also been increased to 250 servers/cloud network:
http://www.rackspace.com/knowledge_center/article/create-an-isolated-cloud-network