Add network model tests

Largely based on the vapp tests, except for explicitly
trying to find a suitable network to test against rather than
just blindly picking the first.

Test lazy loading is working, and that values loaded match
our data from the raw service request.

Lazy loading unfortunately complicates testing as per the
current implementation - determining that the loading has
happened is on a field-by-field basis, which leaves fields
that are undefined in vCloud in a 'NonLoaded' state. Possibly worth
revisiting this logic in a separate commit.
Use admin endpoint for get_network

API provides both api/admin/network and api/network for GET
request of the orgVdcNetwork. The admin endpoint also returns
the attached EdgeGateway, which comes in useful when re-writing
the output to POST back as a new network.
Switch network model over to get_network_complete

This has also fixed an issue where undefined attributes in the
request output were not being unmarked as NonLoaded. Test adjusted
accordingly.

In addition, highlighted that we need the ensure_list! hack on
IpRanges->IpRange.
Add Mock support to get_network_complete

which in turn re-enables Mock support in the network Model