Add deprecation warning for get_network

get_network is being deprecated in favour of get_network_complete,
which returns the complete GET-Network API response from vCloud.

This is a transitional step - see https://github.com/fog/fog/pull/2683

In a future release, get_network will be replaced with the logic from
get_network_complete, and then get_network_complete deprecated
accordingly.

The network Model has been updated to use get_network_complete already.

Any code that relies on existing get_network behaviour should be
updated to use get_network_complete or the Model asap.