Replace Vlan with Network in model.py

Also do trivial fixes to api.py to make this work.

Currently, Network takes a parameter 'nic_id'.  I don't know what this does,
and the API doesn't have a place to specify it.  So, I've set it as 'FIXME'.
This should be fixed.

All tests now pass.
Add REST wrapper for network_{create,delete}
Add CLI wrappers for network_{create,delete}